1 LIGHT HARVESTING POLYMERS: ENERGY TRANSFER AND MATERIALS APPLICATIONS By ZHUO CHEN A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY UNIVERSITY OF FLORIDA 2013
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Light Harvesting Polymers Energy Transfer and Materials Applications
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1
LIGHT HARVESTING POLYMERS: ENERGY TRANSFER AND MATERIALS APPLICATIONS
By
ZHUO CHEN
A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT
OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY
1.2 Photophysical Processes in Light-Harvesting Polymers ................................... 29 1.2.1 Mechanism of Energy Transfer .............................................................. 30
1.2.2 Energy Transfer in Light-Harvesting Polymers ...................................... 32 1.2.2.1 Intermolecular energy transfer .................................................. 33
1.2.2.2 Intramolecular energy transfer .................................................. 34 1.2.2.3 Energy migration in light-harvesting polymers .......................... 35 1.2.2.4 The antenna effect in light-harvesting polymers ....................... 40
1.3 Preparation of Side-Chain Conjugated Polymer: Direct Polymerization of Functional Monomer ......................................................................................... 44
3 ULTRAFAST ENERGY TRANSFER IN POLYSTYRENE BASED ARRAYS OF π-CONJUGATED CHROMOPHORES .................................................................. 114
3.2 Polymer Design and Preparation .................................................................... 115
3.2.1 Preparation of clickable polymer backbones ....................................... 115 3.2.2 Synthesis of Chromophores ................................................................ 117 3.2.3 Preparation of poly-chromophores and model compounds ................. 118
4 TRIPLET-TRIPLET ENERGY TRANSFER IN POLYSTYRENE-BASED PLATINUM ACETYLIDE ARRAYS ....................................................................... 147
5.2 Polymer Design and Synthesis ....................................................................... 185 5.2.1 Synthesis of NMP Initiator ................................................................... 186
5.2.2 Preparation of Polypyridine Ruthenium Functionalized Polymers and and Model Compound ......................................................................... 187
5.3 Absorption and Photoluminescence................................................................ 192 5.4 Amplified Quenching ....................................................................................... 196 5.5 Surface Absorption on Titanium Dioxide Surface ........................................... 198
2-1 Photophysical characteristics of small molecular chromophore, FBPt, and NLA polymer, Poly-FBPt. .......................................................................................... 100
3-1 Photophysical Characteristics of Polymers (P-0 to P-20). ..................................... 131
4-1 Photophysical characteristics of model compounds and poly-platinums. .............. 163
4-2 Liftimes of poly-platinums. ..................................................................................... 169
5-1 The photophysical and electrochemical properties of 2, P1 and P2. ..................... 195
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LIST OF FIGURES
Figure Page
1-1 Examples of π-conjugated polymers. ...................................................................... 25
1-2 Examples of light-harvesting dendrimers ................................................................ 27
1-3 Different morphologies of a polymer film ................................................................. 29
1-4 Comparison of the Förster and Dexter mechanisms of electronic energy transfer. .............................................................................................................. 30
1-5 Types of energy transfer in light-harvesting polymers ............................................. 33
1-6 Superquenching of a dye polymer by energy acceptor ........................................... 34
1-7 Utilization of FRET in biomacromolecules. .............................................................. 35
1-8 A conceptual comparison between dilute solutions of polymer and small molecules ........................................................................................................... 36
1-9 Intramolecular energy exchange in polymers .......................................................... 37
1-10 Possible energy transfer steps in intramolecular migration ................................... 38
1-11 Stern−Volmer plots for emission quenching of Ru-polymer and monomeric Ru
1-12 Emission spectra of the mixture of polystyrene and poly(1-vinylphthalene) and the corresponding copolymer ............................................................................. 41
1-13 Mechanism of the singlet antenna effect. The polymer is a naphthalene substituted polymer, containing an anthracene trap ........................................... 42
1-14 Structure and energy transfer model of the copolymer [co-PS-4- CH2CH2NHC(O)-(RuII)17)(OsII)3](PF6)40. .............................................................. 44
1-15 Example of free radical polymerization. ................................................................ 45
1-16 Side-chain conjugated polymers made by conventional free radical polymerization. ................................................................................................... 46
1-17 Mechanism of living anionic polymerization. ......................................................... 48
1-18 Living anionic polymerization of styrene derivatives para-substituted with π-conjugated oligo(fluorene) moieties. ................................................................... 49
1-19 General concept of controlled radical polymerization (CRP) ................................. 50
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1-20 Mechanism of transition metal catalyzed ATRP. ................................................... 51
1-21 Common nitrogen-based ATRP ligands. ............................................................... 52
1-22 Examples of polymers prepared via ATRP. .......................................................... 53
1-23 Mechanism of the NMP process ........................................................................... 54
1-24 Structures of commonly used nitroxides and alkoxyamines. ................................. 55
1-25 Selected monomers polymerized by NMP for electronic applications. .................. 56
1-26 Various amorphous-crystalline donor–acceptor block copolymers synthesized by NMP ............................................................................................................... 57
1-27 Polymerization of donor-acceptor block copolymers via NMP .............................. 57
1-28 Synthesis of dendronized initiator and subsequent polymer prepared via NMP ... 59
1-29 Block copolymer prepared via NMP for LED application ....................................... 60
1-30 Mechanism of RAFT polymerization ..................................................................... 62
1-31 Structural features of RAFT agents ....................................................................... 63
1-32 Examples of RAFT agents .................................................................................... 63
1-33 Preparation of light-harvesting polymers via RAFT polymerization ....................... 64
1-34 Monomers with pendant functionality polymerized by RAFT and used in optoelectronic applications. ................................................................................ 66
1-35 Chauvin mechanism of olefin metathesis. ............................................................. 67
1-36 Types of olefin metathesis reactions ..................................................................... 67
1-37 General mechanism for ROMP. ............................................................................ 69
1-38 Commonly used olefin metathesis catalysts. ........................................................ 69
1-39 ROMP of a blue-emitting polymer with Mo catalyst ............................................... 70
1-40 Preparation of polymers with pendant Ru complexes by ROMP ........................... 71
1-41 Examples of polymers with pendant metal-complexes prepared via ROMP ......... 72
1-42 Preparation of electroactive polymers via ADMET and post-functionalization ...... 73
1-43 Synthesis of polymers by post-polymerization modifications. ............................... 75
12
1-44 Post-polymerization modification via SN2 reactions .............................................. 76
1-45 Post-polymerization modification via amide coupling ............................................ 77
1-46 Structures of active esters ..................................................................................... 77
1-47 Post-polymerization modification via the reaction between amine and active ester ................................................................................................................... 78
1-48 Examples of metal-coordination reactions in post-polymer modification ............... 79
1-49 Palladium-catalyzed coupling reactions in post-polymerization modification. ....... 81
1-51 Application of CuAAC click reaction in post-polymerization modification .............. 83
1-52 Preparation of copolymers with different functional groups with orthogonal click reactions ............................................................................................................. 84
1-53 Synthetic strategy of side-chain conjugated polymers in this dissertation. ............ 85
2-1 Mechanisms of nonlinear absorption ....................................................................... 87
2-2 Examples of platinum acetylides with TPA/ESA mechanisms.148-149 ....................... 89
2-3 Platinum acetylides used in NLA materials. ............................................................ 89
2-4 Structures of FBPt and Poly-FBPt. ........................................................................ 90
2-5 Synthetic route of FBPt. .......................................................................................... 91
2-6 Synthesis of Poly-FBPt. ......................................................................................... 93
2-7 GPC of polymers ..................................................................................................... 94
2-8 1H NMR spectra of PGMA and PHAZPMA. ............................................................ 95
2-9 Comparison of absorption of PGMA and PHAZPMA. ............................................. 95
2-10 1H NMR spectra of PHAZPMA, Poly-FBPt and FBPt. ......................................... 96
2-11 FTIR spectra of polymers, PGMA, PHAZPMA and Poly-FBPt............................. 97
2-12 Ground state absorption and steady-state emission of FBPt and Poly-FBPt ....... 98
2-13 Transient absorption spectra of FBPt and Poly-FBPt in deoxygenated THF .... 101
2-14 Schematic diagram of a simple open-aperture z-scan apparatus ....................... 102
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2-15 NLA response of 1 mM solutions of blank, FBPt, Poly-FBPt and T2 ................ 104
2-16 Structure of z-scan benchmark, T2. .................................................................... 104
2-17 Photos of Poly-FBPt solution and film under visible and UV light....................... 105
2-18 Ground state absorption and photoluminance of Poly-FBPt thin film. ................ 105
2-19 Transient absorption of Poly-FBPt thin film ........................................................ 106
3-1 Structures of Polymers (P-0 to P-20) and model compounds (1a and 1b). ........... 115
3-2 Synthesis of PVBC and PVBA. ............................................................................. 116
3-3 Synthesis of conjugated chromophores with terminal alkyne. ............................... 117
3-4 Preparation of polychromophores (P-0 to P-20). ................................................... 119
3-5 Synthesis of OPE and TBT model compounds 1a and 1b. ................................... 119
3-6 1H NMR spectra for polymers, PVBC, PVBA and P-0 to P-20. ............................. 120
3-7 GPC traces of polymers ....................................................................................... 121
3-8 Steady-state absorption and emission of model compounds and polymers. ......... 122
3-9 Comparison of measured (brown hollow diamonds) and calculated (cyan hollow stars) absorption spectra of P-20 ...................................................................... 125
3-10 Excitation spectra of copolymers with both donor and acceptor ......................... 126
3-11 Transient absorption spectra showing early time (t = 175 fs) comparison between P-5 and the pure donor polymer P-0. Also shown are transient spectra at t = 1.15 ns comparing P-5 with the TBT acceptor moiety ................ 127
3-12 Transient kinetics from λ = 665 nm for the five polymers .................................... 128
3-13 Molecular dynamics simulation of the OPE-TBT copolymer ............................... 133
4-1 Structures of poly-chromophores (P-0 to P-100) and model compounds. ............. 149
4-2 Preparation of PE2-Pt and Py-Pt chromophores with terminal alkynes (5 and 9). 151
4-3 Preparation of poly-platinums (P-0 to P-100). ....................................................... 152
4-4 GPC traces of Poly-Platinums and PVBA ............................................................ 152
4-5 1H NMR Spectra of Poly-Platinums (P-0 to P-100). .............................................. 155
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4-6 Absorption of platinum acetylides and Poly-Platinums in THF. ............................. 156
4-7 Comparison of measured (brown) and simulated (violet circle) absorption spectra of P-20 ................................................................................................. 158
4-8 Emission of model compounds and poly-platinums in THF ................................... 159
4-9 Quantum yields and energy transfer efficiency for poly-platinum copolymers. ...... 160
4-10 Excitation spectra of poly-platinums .................................................................... 162
4-11 Transient absorption spectra of model compounds ............................................. 164
4-12 Transient absorption spectra of copolymers (P-3 to P-20) at different time and comparison with homopolymers (P-0 and P-100) ............................................. 165
4-13 Phosphorescence decay of poly-platinums at 520 nm ........................................ 167
4-14 Transient kinetics from λ = 600 nm for five polymers P-0 to P-20 on the timescale of 0 to 0.92µs. ................................................................................... 168
4-15 Jablonski Diagram for energy transfer in copolymers ......................................... 170
5-1 Illustration of functional metallopolymer assemblies with controlled pattern adsorbed on photonic electrode ....................................................................... 182
5-2 Polystyrene-based Ru(II) arrays prepared by RAFT polymerization and NMP. .... 183
5-3 Structure of NMP initiator (1), the model ruthenium complex (2) and ruthenium functionalized polymers (P1 and P2). ............................................................... 185
5-4 Synthesis of NMP initiator (1) with triester group. ................................................. 186
5-5 Preparation of polypyridine ruthenium functionalized polymers. ........................... 188
5-6 GPC traces of polymers 7, 8 and 9 ....................................................................... 189
5-7 Synthesis of model Ru(II) complex, 2. ................................................................... 189
5-8 1H NMR spectra of polymes. ................................................................................. 191
5-9 Ground state absorption of model complex and polymers .................................... 192
5-10 Steady-state emission of model complex and polymers ..................................... 193
5-11 Emission quenching of polymers (P1-Cl and P2-Cl) and model complex (2-Cl). 197
5-12 Stern-Volmer plots for emission quenching of P1-Cl, P2-Cl and 2-Cl ................. 198
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5-13 Non-contact tapping mode AFM images of TiO2 (110) with different resolutions and cross-section analysis of C ........................................................................ 199
5-14 Non-contact tapping mode AFM images of P2-Cl deposited on TiO2 (110) surface from solutions of different concentrations. ........................................... 202
5-15 Cross-section analysis for AFM images for Figure 5-17A, B and H. ................... 202
5-16 IPCE spectra for a TiO2 (110) electrode dipped into MeOH with various concentrations of P2 ......................................................................................... 202
5-17 AFM image of the P2 polymer molecules at the surface of TiO2 (110) from solutions of 1 μg/ml for different dipping times ................................................. 203
5-18. Photocurrent action spectrum (IPCE) and J-V curve of DSSC made from P2-Cl and nanocrystalline TiO2. ............................................................................. 205
Abstract of Dissertation Presented to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy
LIGHT HARVESTING POLYMERS: ENERGY TRANSFER AND MATERIALS
APPLICATIONS
By
Zhuo Chen
August 2013
Chair: Kirk S. Schanze Major: Chemistry
Side-chain conjugated polymers combine the intrinsic film-forming and
mechanical properties of polymers and well-defined electronic, photonic, and
morphological properties of monodisperse oligomer moieties. In this dissertation, a post-
SN2 substitution and copper(I) catalyzed azide-alkyne “click” cycloaddition (CRP- SN2-
“click”) is used to prepare side-chain conjugated polymers. Several families of well-
defined light harvesting polymers have been prepared, featuring a non-conjugated and
flexible polymer backbone having desired molecular weight and narrow polydispersity,
and pendant organic or organometallic chromophores.
A polyacrylate with pendant nonlinear absorption (NLA) chromophores was
prepared via the RAFT-SN2-“click” synthetic strategy. Platinum acetylides that undergo
NLA via both two-photon absorption (TPA) and exited-state absorption (ESA)
mechanisms were utilized as chromophores attached to clickable polyacrylate
backbone. The resulting polymer exhibits similar photophysical properties as platinum
acetylide precursor, including steady-state absorption and emission, triplet-triplet
21
transient absorption, and nonlinear absorption properties. In addition, the resulting
polymers can be easily drop- or spin- coated to afford optically transparent film.
Singlet energy transfer along a non-conjugated polymer chain was studied with
graft copolymers having different π-conjugated side groups as energy donor (OPE) and
acceptor (TBT). The graft copolymers were also prepared from the reversible adition-
fragmentation transfer polymerization (RAFT)-SN2-“click” route. The singlet energy
transfer from donor to accept was characterized employing both time-resolved and
steady-state fluorescence spectroscopy, as well as ultrafast time-resolved transient
absorption spectroscopy. The ultrafast energy transfer from OPE to TBT occurs within
50 picosecond with remarkably high efficiency. There were two energy migration
processes existing: ultrafast neighboring OPE-TBT quenching within 2 - 4 ps and OPE-
OPE hopping within 25 - 50 ps.
In a similar approach, the triplet energy transport has also been studied in Pt-
acetylide chromophore arrays that were assembled by polystyrene backbone. When the
click reaction is carried out, a low energy “trap” (pyrene-Pt) was doped into the
assembly along with high energy PE2-Pt on the polymer backbone. The triplet-triplet
energy transfer from PE2-Pt to Py-Pt was found to be very efficient, occurring within 50
ns.
Finally, a ruthenium(II)-functional polymer with carboxylic acid bearing end-group
was prepard via nitroxide-mediated polymerization (NMP)-SN2-click strategy. The
polymer shows typical absorption and emissiton as Ru(bpy)32+ complex and also
exhibits amplifed quenching effect. Absorption of the acid-end-group polymer onto
single crystal TiO2 has been characterized by AFM and photocurrent action spectra.
22
The results indicate Ru-functional polymer is able to anchor to TiO2 surface with the
acid end-groups and inject electrons to TiO2 to produce current. DSSCs made from the
acid-end-group Ru-functional polymer and nanocrystalline TiO2 shows photon-to-current
conversion with a low overall efficiency. Further efforts, including tuning polymer chain
length, adjusting structures of pendant ruthenium complex and optimizing DCCS
fabrication technique, etc., should be made to improve the solar cell performances.
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CHAPTER 1 INTRODUCTION
1.1 Light-Harvesting Systems
Light-harvesting systems bear repetitive chromophores which can absorb
photons to mimic the natural photosynthetic approaches.1-2 Multichromophoric light-
harvesting systems have attracted a lot of interest in the research communities of
chemistry, materials science and chemical engineering. There are several reasons for
the attention paid to light-harvesting systems. First, they are capable of converting sun-
light energy into electrical energy, for example, they can be used in solar cells.3 Second,
these systems can also act as catalyst to convert solar energy into easily storable
chemical energy, for example, molecular hydrogen by water splitting, hydrocarbons,
methanol and formic acid by water reduction of CO2.3-8 Third, they make valuable
contributions to light/electro-responsive materials such as organic light-emitting devices
(OLEDs),9-12 optical data storage13-14 and optical limiting.15
Among the structures of light-harvesting systems, light-harvesting polymers are
particularly attractive because numerous synthetic methods make it possible to
construct all kinds of polymeric architectures. Also, from a structural point of view, light-
harvesting polymers can be grouped into three categories, polymers with π-conjugated
backbones (normally known as conjugated polymers),16 dendrimers17 and polymers with
a non-conjugated backbone (e.g., polystyrene and polyacrylate) but pendant π-
conjugated chromophores as side-chains (also termed as side-chain conjugated
polymers).2, 18 The major topic in this dissertation will be side-chain conjugated polymers,
including their preparation, characterization and application.
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1.1.1 Conjugated Polymers
In general, the electronic structure of π-conjugated polymers originates from the
sp2pz hybridized wave functions of the carbon atoms in the repeat units. The π-cloud of
one monomer is in conjugation with all other repeat units around it. This extended
conjugation lowers the energy required to promote an electron from the highest
occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO)
of the conjugated units.16 In conjugated polymers, such as poly(3-hexylthiophene)
(P3HT) and polyfluorene (PF), the backbone provides the light-harvesting function.
Such materials exhibit good processability, extraordinarily high extinction coefficients,
and the possibility to tune optical gaps and HOMO and LUMO energies. Some
representative examples of conjugated polymers are shown in Figure 1-1.16
However, the fully π-conjugated polymer chain was broken into a series of
segment chromophores with different conjugation lengths because of conformational
disorder. The conjugation difference leads to exciton self-trapping, which limits exciton
diffusion lengths to ~10 nm, and results in low fluorescence quantum yields.19-27 From
the synthetic point of view, it is difficult to control the chain length of conjugated
polymers, which have broad molecular weight distribution. Thus, it makes the
reproducibility from batch to batch to be very poor.
25
Figure 1-1. Examples of π-conjugated polymers. Adapted with permission from ref. 16. Copyright 2010 The Royal Society of Chemistry.
1.1.2 Light-Harvesting Dendrimers
Dendrimers are perfectly highly branched synthetic macromolecules having three
architectural components: a central core, interior branches, and surface functional
groups. Dendrimers have attracted great scientific interest because of their unique
molecular architecture.17 Dendrimers are symmetric and monodisperse molecules and
are synthesized through a stepwise repetitive reaction sequence, which gives rise to
26
different generations of the dendrimers. Such distinguished frameworks induce
relatively rigid conformation in terms of size and shape compared to linear polymers.
Dendrimers featuring decreasing number of chromophores from periphery to
core make an attractive candidate for light-harvesting applications. The dendron acts as
a well-defined scaffold to link chromophores and restricts the position and distance
between different chromophores so that the energy transfer from periphery to core can
be “oriented”. Numerous dendritic designs with different kinds of light-collecting
chromophores at periphery and an energy-sink at the core have demonstrated high
energy transfer efficiency, and an example is presented in Figure 1-2A.28 The scaffold of
dendrimers can also be π-conjugated; in this case the conjugated dendrons act both
light-absorbing chromophores and delocalized energy/electron transfer pathways
(Figure 1-2B).29-30 Because of the precise structure and high energy transfer efficiency,
light-harvesting dendrimers are now being developed for applications such as light-
emitting diodes, frequency converters and other photonic devices.28-30
The disadvantage of dendrimers lies in the difficulty of synthesis. The step-by-
step synthesis leads to large amount of work. With the increasing size in the higher
generation, the reactivity of reacting sites become lower, leading to low yields of high
generation dendrimer products; meanwhile, it also brings difficulty of purification.
27
Figure 1-2. Examples of light-harvesting dendrimers. A) Dendrimers with non-conjugated scaffold. Reproduced with permission from ref. 28. Copyright 2000 The Royal Society of Chemistry. B) Dendrimers with conjugated scaffold. Reproduced with permission from ref. 30. Copyright 2006 Springer.
1.1.3 Side-Chain Conjugated Polymers
From the perspective of structure, side-chain conjugated polymers are
considered to be polymers with a non-conjugated backbone (e.g., polystyrene and
polyacrylate) and pendant π-conjugated organic or organometallic chromophores.2 The
research of such polymeric systems was pioneered by Fox, Guillet, Webber and
coworkers to mimic the process of natural photosynthesis,1-2, 31 from which the term
“light-harvesting polymer” originally came.
For main-chain conjugated polymers the optoelectronic properties depend
strongly on the length of the conjugated system,24, 32 while in side-chain conjugated
polymers the electronic properties of the individual chromophores are mainly unaffected.
Therefore, the side-chain conjugated polymers combine the typical polymer properties
(e.g., film-forming ability, mechanical stability, and processing advantages) with the
well-defined electronic, photonic, and morphological properties of the monodisperse
28
oligomer moieties. In addition, the generally low solubility of the oligomers can be
improved significantly by their incorporation to a polymer structure.33-35
With the development of controlled radical polymerization (CRP) and living
polymerization techniques, side-chain conjugated polymers can be obtained with
precisely controlled polymer length, which means excellent reproducibility of side-chain
conjugated polymers from batch to batch.31,32,36
Because of the controllability of CRP and living polymerization, it is practical to
prepare random copolymers and block copolymers bearing different chromophores.
There are a lot of variables for the copolymer structure allowing one to tune the
properties of the copolymers and devices based upon them. First of all, the
chromophores installed into the polymer side chains can be adjusted and optimized for
different properties. For example, energy donor and acceptor can be attached to the
polymer backbone at the same time to induce energy transfer.34, 37
Secondly, by tuning relative concentrations of different chromophores, the
properties of copolymers and devices can also be adjusted. Fréchet et al. reported that
by tuning the concentration of blue emissive platinum complex in a random terpolymer,
a high external quantum efficiency of 4.6% can be achieved in the polymer-based white
OLED.10
Finally, the morphologies of polymer films can be controlled by adjusting relative
length of different blocks in block copolymers to achieve different goals for different
types of devices. The possible morphologies of polymer layer, polymer blends and
copolymers are illustrated in Figure 1-3. For copolymers, the domain size can be much
smaller than polymer blends. However, the domain sizes in copolymer films can still be
29
adjusted. It can be adjusted to be in the range of around 15 nm to meet the
requirements for polymer solar cells (equal to exciton diffusion length);38-40 or it can fall
into the range of 26 to 49 nm to suppress the energy transfer between different emitting
chromophores to achieve the “site-isolation” effect, which meets the requirements for
white LEDs.9
Figure 1-3. Different morphologies of a polymer film. Adapted with permission from ref. 40. Copyright 2006 John Wiley & Sons.
In this dissertation, the main focus will be on the side-chain conjugated polymers,
and light-harvesting polymers will be particularly referred to the side-chain conjugated
polymers. This chapter will continue to discuss the energy transfer in side-chain
conjugated polymers and the preparation strategies of such polymers.
1.2 Photophysical Processes in Light-Harvesting Polymers
Many kinds of light-harvesting polymeric assemblies have been made, with one
or more chromophores covalently attached to a single polymer strand. The
chromophores attached to the backbone can be the same or different. In polymers with
different chromophores, often one acts as light-absorbing sensitizer, or energy donor
(as it often has higher excited state energy), and another chromophore acts as
quencher (or energy acceptor, as it often has lower excited state energy). Energy
30
transfer plays an important role in the properties and applications in light-harvesting
polymers, and this is the photophysical processes people of most interested.
1.2.1 Mechanism of Energy Transfer
An electronic energy transfer process can be simply described by Equation 1-1:
*D + A D + *A (1-1)
There are two different mechanisms for energy transfer. The first case is the
electron exchange interaction (also known as “orbital overlap mechanism” or “electron
exchange mechanism”) which is often referred as Dexter energy transfer (Figure 1-4A);
and the second case is the dipole-dipole interaction (also termed as the “Columbic” or
“resonance” mechanism for electronic energy transfer in the literature) which is known
as Förster energy transfer (Figure 1-4B).
Figure 1-4. Comparison of the Förster and Dexter mechanisms of electronic energy transfer. A) Dexter (exchange) mechanism. B) Förster (dipole-dipole,) mechanism. The spin of the electrons exchanged must obey the spin conservation rules. Adapted with permission from ref. 71. Copyright 2009 University Science Books.
31
In Figure 1-4, the interacting electrons were labeled as 1 and 2. A key difference
between the two mechanisms is that, for the Förster mechanism, the interaction
between *D and A is through space by the interaction of dipolar electric fields of *D with
A, while for the Dexter mechanism, the interaction between *D and A is through the
overlap of the orbitals of *D and A. The dipole-dipole interaction operates through an
oscillating electric field produced by *D and does not require a van der Waals contact of
*D and A or an overlap of the orbitals for *D and A. From Figure 1-4A, it is seen that
electrons 1 and 2 exchange positions between A and D for electron transfer; whereas
from Figure 1-4B, it is seen that electron 1 stays on D and electron 2 stays on A.41
The rate of energy transfer (kEnT) of the Dexter mechanism can be expressed as:
DAEnT 0
DA
2(Dexter) exp( )
Rk KJ
R
(1-2)
Where K is a parameter related to the specific orbital interactions, J is the
normalized spectral overlap integral, and 0
DAR is the separation of *D and A when they
are in van der Waals contact. From the equation, the exponential dependence of the
efficiency on the RDA (distance between D and A) ensures that energy transfer is only
efficient over fairly short distances on the order of RDA = 5-10 Ǻ. So the Dexter
mechanism requires donor and acceptor to be very close to each other, if not in direct
contact.41
On the other hand, the rate of energy transfer (kET) of the Förster mechanism is
proportional to the inverse sixth power of the separation between *D and A:
2 2
D AET 6
DA
(Forster)kR
(1-3)
32
According to the equation in favorable cases the range of separation for *D and A
energy transfer by the dipole-dipole mechanism can be very large, in the order of RDA >
30 Ǻ.41
1.2.2 Energy Transfer in Light-Harvesting Polymers
Energy transfer between groups attached to a polymer backbone occurs by the
two mechanisms described in the previous section; however, it is more complicated
than the bimolecular energy transfer occurring between small molecules. Three major
types of energy transfer in a polymer system can be distinguished: intermolecular
energy transfer between polymer and small molecule, intramolecular energy transfer
and energy migration, as presented in Figure 1-5.31
Intermolecular energy transfer involves the transfer of excitation energy from or
to a small molecule from a large molecule. The exciton can be originally localized on a
small molecule (donor), and transfer to a polymer chain, causing a sensitized
photochemical reaction. Alternatively, the exciton can initially be localized on a
chromophore on a polymer chain and transfer to a small molecule (acceptor), thus
quenching a photochemical or photophysical process (as shown in Figure 1-6A). The
second type of energy transfer in light-harvesting polymers, which is of special interest
to polymer chemists, is energy transfer between chromophores in the same polymer
chain. In this case, an exciton localized in donor group can be quenched by an acceptor
in the same polymer chain (i.e., intramolecular energy transfer, as presented in Figure
1-6B). Additionally, exciton localized on a sequence of chromophores may be
transferred from one chromophore to the next by a hopping mechanism in a process
that is termed energy migration (Figure 1-5C). Although these energy transfer
33
processes are described separately in convenience, two or three energy transfer
mechanisms normally co-exist in a polymeric system.31
Figure 1-5. Types of energy transfer in light-harvesting polymers. A) Energy transfer
between polymer and small molecule. B) Intramolecular energy transfer. C) Energy migration. Adapted with permission from ref. 31. Copyright 1985 John Wiley & Sons.
1.2.2.1 Intermolecular energy transfer
An example of energy transfer from polymer to small molecule is that exciton
from polymer is quenched by energy acceptor quenchers.42-43 In light-harvesting
polymers with multiple chromophores, the quenching process also involves energy
migration along the polymer chain, resulting in a much more pronounced quenching
effect compared to that of monomeric model compound. This phenomenon is termed
“amplified quenching” or “superquenching”. Whitten et al.42 illustrated quenching of a
polymer with pendant cationic cyanine dyes on a L-lysine “scaffold” (P in Figure 1-7A)
by a negative charged cyanine (A in Figure 1-7A). The anionic cyanine dye (A)
quenches the fluorescence of P at very low concentrations and gives a very high Stern-
Volmer constant (KSV) of 4 x 107 M-1. In addition to decreasing intensity of fluorescence
34
peak of P, a new fluorescence peak at 630 nm appeared, which was assigned to o a
“complex” form of A that is selectively activated by an energy transfer process from
photoexcited P.
Figure 1-6. Superquenching of a dye polymer by energy acceptor. A) Structures of the
dye polymer P and acceptor A. (b) Absorption and emission spectra of dye polymer P upon sequential addition of cyanine acceptor A: solid line, polymer P absorption; dotted line, polymer P emission with no dye A added; dot-dash and dashed lines, emission spectra recorded on sequential addition of A. Adapted with permission from ref. 42. Copyright 2001 American Chemical Society.
1.2.2.2 Intramolecular energy transfer
Intramolecular energy transfer means energy transfer from one chromophore to
another in the same polymer chain. An important example is Förster resonance energy
transfer (FRET). A donor and an acceptor can be installed at both α- and ω-ends of a
polymer, and then FRET between the donor and acceptor can act as the probe to study
end-to end distance, chain dynamics and conformation.44-47 Zentel et al. utilized
reverse addition-fragmentation transfer (RAFT) polymerization to prepare α, ω dye-
35
functionalized polymers with narrow molecular weight distribution, in which Oregon
Green Cadaverin served as donor and Texas Red acted as acceptor. The calculated
end-to end distance of the polymer with FRET was in reasonable agreement with data
obtained from light scattering and gel permeation chromatography.45
FRET is very useful for the characterizations of biomacromolecules, such as
peptides and nucleic acids (Figure 1-8)46 As a probe, it is used to study peptide/DNA
length and conformational distributions. By FRET, the length of rigid
biomacromolecules, such as rigid peptides and double-strand DNAs, can be accurately
determined; and the dynamical properties of flexible peptides can also be determined.
Figure 1-7. Utilization of FRET in biomacromolecules. Adapted with permission from ref. 46. Copyright 2001 American Chemical Society.
1.2.2.3 Energy migration in light-harvesting polymers
In the cases of a polymer bearing multiple chromophores along the polymer
chain, the energy transfer behavior will be much more complicated compared to the
polymers described in the previous section. Polymers enhance absorptivity by
increasing the number of sensitizers (usually act as donor) bound to a polymer
backbone.
36
A polymer with pendant chromophores is illustrated in Figure 1-8. Even though
the polymer solution is diluted, the “concentration” of groups of chromophores along the
polymer backbone remains relatively constant. Additionally, the distance between
neighboring chromophores will be determined by the geometry and flexibility of the
polymer chain. This is an important factor that differentiates between inter- and
intramolecular energy transfer, since intermolecular transfer depends on the polymer
concentration, while intramolecular processes are relatively independent.31
Figure 1-8. A conceptual comparison between dilute solutions of polymer and small molecules. A) Dilute polymer solution. B) Dilute smaller molecular solution. Adapted with permission from ref. 31. Copyright 1985 John Wiley & Sons.
In a molecule where the chromophores are sufficiently close to each other, the
transfer of excitons from one chromophore to another can be viewed as a random walk
on the chromophores. There are mainly three possible types of intramolecular energy
exchange, as shown in Figure 1-9: (1) the hopping of exciton from one chromophore
group to another, often adjacent group along the polymer backbone (Figure 1-9A); (2)
movement of exciton along the conjugated backbone by an exciton band mechanism
37
(Figure 1-9B); and (3) movement of exciton across loops in a single polymer chain,
which could be formed by folding of polymer chain to cause a temporary collision
(Figure 1-9C).
Figure 1-9. Intramolecular energy exchange in polymers. Adapted with permission from
ref. 31. Copyright 1985 John Wiley & Sons.
In principle, the exciton can be localized on a particular chromophore group, at
least for some finite period of time, before it moves to another group in the chain. This
type of energy delocalization is referred as “energy migration”. And normally a single
step between chromophore unites is termed “energy transfer”, while more than one
such step in sequence constitute “energy migration”. Three distinct types of energy
transfer steps can contribute to energy migration in a polymer chain, which are
illustrated in Figure 1-10. The first one is the “nearest neighbor” transfer, which is
defined as transfer between chromophores where n, the number of monomer units
between those bearing the transfer sites, is equal to zero. This is important in polymers
containing small chromophores with a flexible backbone.31
38
The second type is “non-nearest neighbor” transfer, where n = 1, 2, 3. This type
of energy transfer occurs with chromophores which are prohibited by steric or structural
effects from approaching an adjacent chromophore by facile bond rotation. This type of
polymer often has large substituent chromophore groups attaching to backbone. In
order to achieve the most stable conformation, large chromophores will separate as far
apart as possible, thus may preclude nearest neighbor interactions, especially when the
lifetime of the excited states is shorter than the rotational relaxation time of the polymer
chain and side groups. In this case, the interposition of a single repeat unit (n=1) may
give the closest approach of two chromophores.31
Figure 1-10. Possible energy transfer steps in intramolecular migration. A) Nearest neighbor transfer (n=0). B) Non-nearest neighbor transfer (n=1, 2, 3). C) Loop transfer (n>3). n is the number of monomer unites between those bearing the transfer sites. Adapted with permission from ref. 31. Copyright 1985 John Wiley & Sons.
The third type of process is termed “loop transfer” (n>3). It happens when there is
strong solvent effect on the polymer conformation. And it may also happen when the
polymer chain is long enough to allow the polymer to fold, thus two chromophores with
n>3 may approach close enough to exchange excited state energy.
39
Experimental detection of energy migration includes quenching the polymer with
small molecules. If there is energy migration, the Stern-Volmer constant will be much
larger than that of quenching of small molecular chromophores. This phenomenon is
termed “amplified quenching” or “superquenching”. This concept was first studied by
Swager and coworkers in main-chain conjugated poly(phenylene ethynylene)s
(PPEs).48-49 Whitten, Schanze and coworkers studied the amplified quenching effect
extensively in conjugated polyelectrolytes (CPEs). CPEs can be quenched by small
amount of oppositely charged quencher ions. This process has been attributed to two
main factors: (1) ion-pairing between the (oppositely) charged quencher ion and repeat
units in the polyelectrolyte chain effectively increases the local concentration of the
quencher ion, and possibly more important, (2) the fact that excitons in the poly-
chromophore are able to undergo rapid diffusive transport along the polymer chain,
increasing the effective sphere of action of the quencher ion.50 These two factors can
also be used in the light-harvesting side-chain conjugated polymers, which have a non-
conjugated backbone.51-52
Schanze and coworkers studied the amplified quenching effect in a polymer with
pendant ruthenium complexes and a polystyrene backbone (Figure 1-12). The polymer
was quenched by 9,10-anthraquinone-2,6-disulfonate (AQS), an electron acceptor. For
polymer with 20 repeat units, the Stern-Volmer constant is 15 times larger than the
monomeric model Ru complex (8.7 x 105 M-1 vs. 6.3 x 104 M-1). As the polymer length
increased, the Stern-Volmer constant also increased.52 These experimental results
clearly indicate that even with a nonconjugated polymer backbone, excited energy can
40
migrate efficiently. Calculation of similar structure reveals than the energy migration is
site-to-site through-space hopping.8, 53-54
Figure 1-11. Stern−Volmer plots for emission quenching of Ru-polymer and monomeric
Ru complex. Adapted with permission from ref. 46. Copyright 2012 American Chemical Society.
1.2.2.4 The antenna effect in light-harvesting polymers
In 1969, Fox et al. found that efficient phosphorescence emission occurred from
small amounts of copolymerized chemically bond energy traps in polymer chains
(styrene-vinylnaphthalene copolymer). Emission of naphthalene phosphorescence from
the copolymers was much higher than that of the mixture of equivalent amounts of the
two homopolymers in solution (Figure 1-13).55-56 Later, Schneider and Springer found
the similar phenomenon in styrene-acenaphthalene copolymer.57 The enhanced
acceptor emission was explained to be due to energy migration along the polymer
chain. Because it mimics that observed in the ordered chlorophyll regions of green plant
chloroplasts, i.e., the antenna chlorophyll pigments, this effect is termed to be the
“antenna effect”.31
41
Figure 1-12. Emission spectra of the mixture of polystyrene and poly(1-vinylphthalene)
and the corresponding copolymer. Adapted with permission from ref. 55. Copyright 2012 American Chemical Society.
Guillet et al. studied singlet energy migration and transfer in a variety of
copolymers containing naphthalene and phenanthrene donors with anthracene energy
traps, and proposed that the antenna effect for singlet energy transfer was not
exclusively due to energy migration among chromophores making up to the antenna but
to a combination of energy migration and direct Förster transfer to the acceptor (trap),
as illustrated in Figure 1-14. Assuming only one trap occurs in a long antenna chain, on
a short time scale, i.e., a typical singlet lifetime (τ < 100 ns), collisional energy transfer is
a relatively minor factor, as the conformational relaxation time is much longer than the
singlet lifetime. Thus energy migration and transfer should be dominated by the long-
range Förster dipole-dipole mechanism.31
42
Figure 1-13. Mechanism of the singlet antenna effect. The polymer is a naphthalene
substituted polymer, containing an anthracene trap. Adapted with permission from ref. 31. Copyright 1985 John Wiley & Sons.
In Guillet’s theory, the radius RF defines a sphere around the acceptor inside of
which the direct Förster transfer from the absorbing chromophore to the trap is favored.
Outside of RF, at least one energy migration step will occur before the energy is trapped
by the acceptor. The radius R0 is the standard Förster radius. Although a substantial of
donor-donor transfer may occur in this region, the energy would be transferred to trap in
any case, as long as the radius was less than the Förster radius. So the donor-donor
energy migration in this region would not be expected to contribute to the efficiency of
energy collection. The only increase in efficiency of energy collection due to energy
migration process lies in the region between R0 to RN. In this region, the energy
migration occurs between the donors at a rate corresponding to that in the absence of
the acceptor (energy trap). In addition, the lifetime of singlet exciton allow it has
sufficient time to hop into the R0 radius and finally transfer the energy to the acceptor.
43
Outside of RN, the hopping distance, or hopping steps, is too large for the singlet exciton
to transfer to the acceptor before it relaxes back to the ground state of donor.31
As the optical transitions from ground state (S0) to the lowest triplet state (T1) are
spin-forbidden, the Förster dipole-dipole mechanism is excluded in the triplet energy
migration and energy transfer. As a matter of fact, the short-range Dexter electron
exchange mechanism works in the triplet energy migration and energy transfer.41
Therefore the mechanism of triplet antenna effect is much simpler than that of the
singlet energy transfer, which is mainly due to the triplet energy migration along the
polymer chain.
Meyer, Papanikolas and co-workers studied the energy transfer in the copolymer
based on polystyrene backbone and with fully loaded Ru(II) and Os(II) metal
complexes, [co-PS-4-CH2NHC(O)-(RuII)17)(OsII)3](PF6)40. The Monte Carlo simulation
illustrated the structural influence of the large excluded volumes of the complexes
resulting in rod-like, spatially extended structures, in which the transition metal complex
cations are presented with large spheres with 14 Ǻ diameter. Each complex has 4 to 5
nearest neighbors, and the average distance between peripheries is 2 - 3 Ǻ. The rate
constant of <kEnT> ~ 2.5 x 109 s-1 for the nearest neighbor RuII* to OsII energy transfer
and <kmig> ~ 2.5 x 108 s-1 to 1 x 109 s-1 for RuII* to RuII energy migration (lifetime of 1 – 4
ns, 50 times faster than RuII* excited state decay) were observed. Experimental results
and Monte Carlo simulations conclude that the intrapolymer energy transfer quenching
involves a combination of random walk, energy migration (kmig), and energy transfer
(kEnT) events. Over 80% of the energy transfer quenching events utilize one or more
RuII* to RuII energy migration steps with contributions to energy transfer from pathways,
44
in which there are at least 100 migration steps. It is demonstrated that the 2 - 3 Ǻ
average periphery-to- periphery distance between nearest neighbors in the polymer is
sufficient to promote facile through-space energy migration and energy transfer.8, 54, 58
Figure 1-14. Structure and energy transfer model of the copolymer [co-PS-4- CH2CH2NHC(O)-(RuII)17)(OsII)3](PF6)40. (a) Chemical structures of the copolymer and ligands. (b) Molecular structure of the copolymer from a Monte Carlo simulation. (c) Model for the energy transfer quenching. Adapted with permission from ref. 54 and 55. Copyright 2001 and 2002 American Chemical Society.
1.3 Preparation of Side-Chain Conjugated Polymer: Direct Polymerization of Functional Monomer
1.3.1 Conventional Free Radical Polymerization
The use of macromolecular structures for the assembly of arrays of
chromophores attached to a single polymer backbone was pioneered by Fox et al.55 and
Schneider et al.57 They prepared polymers with naphthalene chromophores by
45
copolymerization of styrene and vinylnaphthalene or acenaphthalene using free radical
polymerization. This kind of light harvesting polymer was further studied by Guillet,59
Webber and many other researchers,2 with chromophores such as biphenyl,
phenanthrene, carbazole, pyrene, anthracene, etc. Synthetically, these polymers were
prepared via conventional free radical polymerization, either by homopolymerization of
chromophore-containing vinyl monomers, or copolymerization of chromophore-
containing monomers and other monomers, such as styrene and methacrylates. The
advantage of free radical polymerization is that this technique has been well-developed
and easy to handle.
Figure 1-15. Example of free radical polymerization.
While the earlier works of preparing side-chain conjugated polymers were
focused on the photophysical studies, efforts were also made to the device application.
Due to the easy solution-processing of side-chain conjugated polymers, they are used
to prepare single- and multi-layer LED devices.60-64 Vinyl groups can be installed onto
functional chromophores in several ways. The functional chromophores can be
converted to urethane/methacrylate or urea/methacrylate monomer by reaction between
amine groups on chromophores and isocyanate group of 2-isocyanatoethyl
As a living polymerization, anionic polymerization was utilized to prepare side-
chain conjugated polymers with controlled molecular weight and narrow molecular
weight distribution. Hirao, Chen and coworkers employed anionic polymerization to
prepared a family of homo-and copolymers of polystyrene with pendant π-conjugated
oligo(fluorine) chromophore moieties with PDI less than 1.08, and the molecular weight
of the polymers were ranging from 3500 to 72400 g/mol by varying the
monomer/initiator ratio.71
Figure 1-18. Living anionic polymerization of styrene derivatives para-substituted with π-conjugated oligo(fluorene) moieties. Adapted with permission from ref. 71. Copyright 2009 American Chemical Society.
50
The photopolymer with different oligofluorene length were used to fabricate non-
volatile memory devices.72 Hirao, Chen and coworkers reported different oligofluorene
length affected the turn-on threshold voltages. Meanwhile, different surface
morphologies of polymers were achieved by using good solvent (chlorobenzene, CB)
and mixture of good/poor solvents (CB/DMF). The results showed the polymer thin film
from CB/DMF mixture gave larger polymer aggregation domains, which promoted the
diffusion of the Al atoms into the polymer film and formed the conduction channel and
significantly reduced the turn-on threshold voltage on the studied polymer memory
devices. The polymer memory characteristics could be efficiently tuned through the
pendent conjugated chain length and surface structures.72
1.3.3 Controlled Radical Polymerization
Controlled radical polymerization, or living radical polymerization, has been
achieved by minimizing normal bimolecular termination and prolonging the lifetime of
living polymers into hours or longer through the introduction of dormant states for the
propagating species (Figure 1-20). This is achieved through alternate modes of reaction
for the propagating radicals, specially, by either reversible termination (ATRP and NMP)
or reversible transfer (RAFT).65
Figure 1-19. General concept of controlled radical polymerization (CRP). Reproduced with permission from ref. 73. Copyright 2013 Elsevier.
51
1.3.3.1 Atom transfer radical polymerization (ATRP)
Since the initial discovery in 1995,66-68 atom transfer radical polymerization
(ATRP) has become one of the most powerful and robust CRP techniques, which result
in unprecedented control over the preparation of many new well-defined (co)polymers
with predictable molecular weight (MW) and narrow molecular weight distribution
(MWD).73-74
Figure 1-20. Mechanism of transition metal catalyzed ATRP. Adapted with permission from ref. 76. Copyright 2009 American Chemical Society.
The ATRP mechanism75-77 (Figure 1-21) involves homolytic cleavage of the alkyl
(pseudo)halogen bond (R-X) by a transition metal/ligand complex in its lower oxidation
state (MnY/Lm) to generate the corresponding higher transition metal complex
(XMn+1Y/Lm) and an alkyl radical (R.), with a rate constant of kact. The resulting alkyl
radicals (R.) initiate the polymerization by adding across the double bond of a vinyl
monomer. Once the polymerization is initialized, the radicals propagate (kp), and
terminate by coupling or disproportionation (kt), or by reversibly deactivated in this
equilibrium by XMn+1Y/Lm (kdeact). In a well-controlled ATRP system, radical termination
52
is diminished as a result of the persistent radical effect (PRE)78-79 that strongly shifts the
equilibrium towards the dormant species R-X (i.e., kact<< kdeact).
Efficient ATRP employs the transition metal/ligand complex as the catalyst to
obtain good control over the molecular weight and MWD. The majority of publications
on ATRP deal with Cu-mediated process. Ligands for Cu to form active and inexpensive
catalytic complexes have been developed, most of which are nitrogen-based.75, 80 The
commonly used ligands are summarized in Figure 1-22.81 One advantage of ATRP is
that all ATRP reagents, including initiators, copper salts and ligands are commercially
available and relatively inexpensive.
Figure 1-21. Common nitrogen-based ATRP ligands.
Zhao et al.82 reported the application of ATRP in the preparation of a series of
liquid crystalline diblock copolymers composed of a polystyrene block and a
polymethacrylate block with an azobenzene moiety in the side chain, and the PDI was
controlled to be less than 1.3. The homopolymer of azobenzene-containing polymer
PAzo had also been prepared with a PDI of 1.23; however, the molecular weight of
PAzo is much higher than the calculated value based on the feed ratio of
53
monomer/initiator. One possible explanation proposed by the authors was that part of
the initiator was not active in inducing polymerization of the azobenzene monomer.
Lin et al.83 synthesized side-chain conjugated polymer with 4, 4’-bis(biphenyl)
fluorene pendants with commercialized ethyl 2-bromo-2-methylpropanoate initiator and
polystyrene macroinitiator. All polymers were obtained with PDIs less than 1.30. The
structures of the polymers are shown in Figure 1-22.
Figure 1-22. Examples of polymers prepared via ATRP.
However, ATRP has its own drawbacks. First, the monomer/initiator/copper ratio
needs to be optimized for each monomer. Second, the tolerance for monomer is not
good enough. Some nitrogen-bearing monomers, halogen-bearing monomer and some
other monomers cannot be polymerized by ATRP. Third, as copper is involved in the
polymerization system, it may affect device performance of the resulting polymer.65
1.3.3.2 Nitroxide-mediated polymerization (NMP)
Nitroxide-mediated polymerization (NMP)84-89 is another CRP technique based
on the reversible termination mechanism. The growing propagating (macro)radical is
terminated by the nitroxide, acting as a control agent, to yield a (macro)alkoxyamine as
the predominant species. This dormant (macro)alkoxyamine functionality generates
54
back the propagating radical and the nitroxide by a simple homolytic cleavage upon
temperature increase. The activation-deactivation equilibrium between dormant and
active species is established. Unlike ATRP or RAFT, this equilibrium takes the
advantage of being a purely thermal process with neither catalyst nor bimolecular
exchange being required. The polymerization kinetics is controlled by both the
activation-deactivation equilibrium (with an activation-deactivation constant K=ka/kd) and
the persistent radical effect (PRE).89
Figure 1-23. Mechanism of the NMP process. Reproduced with permission from ref. 73. Copyright 2013 Elsevier.
The initiation system of NMP can be divided into two categories. The first
contains a conventional thermal initiator, such as 2,2’-azobisisobutyronitrile (AIBN) or
benzoyl peroxide (BPO), and a stable free nitroxide radical such as 2,2,6,6-
tetramethylpiperidinyl-1-oxy (TEMPO)85. In this bimolecular system, conventional radical
polymerization process conditions are employed with the only additions of free
nitroxides, which is both economic and practical desirable. However, fine-tuning of the
55
nitroxide/initiator ratio is required to control the polymerization kinetics for each
polymerization system.
Rizzardo84 and Hawker88, 90-92 developed the concept of unimolecular initiator,
alkoxyamine, the second initiation system. It decomposes into both the initiating radical
and the nitroxide. Upon thermal dissociation, 1: 1 initiating radical and the nitroxide
releases and shows better control over molar masses and MWDs than bimolecular
systems. Another advantage of alkoxyamine is that the structure can be tuned to allow
advanced macromolecular synthesis or post-modification. The most used alkoxyamines
are SG1-based and TIPNO-based initiators. The SG1-based alkoxyamine BlocBuilder®
(also referred as MAMA-SG1) was commercialized by Arkema in kilogram scale in 2005
and TIPNO-based initiators have been commercialized by Sigma-Aldrich recently.88 The
structures of common used nitroxides and alkoxyamines are presented in Figure 1-25.
Figure 1-24. Structures of commonly used nitroxides and alkoxyamines.
The alkoxyamines need high temperature, normally around 120oC, to dissociate
to initiating radicals and nitroxides, it may cause some side reactions during the
polymerization. However, the monomer tolerance of NMP is much better than that of
ATRP. Vinylbenzyl halide and 1,3-dienes can also be polymerized by NMP. NMP
controles the polymerization of styrene and styrene derivatives best among the
publications; however, it shows low controllability over methacrylates monomers, due to
56
slow combination of nitroxides with sterically hindered propagating poly(methacrylate)
radicals and degradation of poly(methacrylate) radicals via β-hydrogen abstraction by
nitroxides.88
As there is no metal catalyst existing in NMP systems, NMP has been used in
preference to ATRP for polymerization of monomers which are intended for electronic
application, such as arylamine-functionalized monomers, oxadiazole, and carbozoles
(Figure 1-26).88
Figure 1-25. Selected monomers polymerized by NMP for electronic applications. Reproduced with permission from ref. 89. Copyright 2013 Taylor & Francis.
Thelakkat et al.38-40, 93 prepared a series of donor-acceptor block copolymers using
NMP. These polymers contain a poly(vinyltriphenylamine) segment (PvTPA) as hole-
transport block and a polyacrylate segment bearing perylene bisimide side groups as
electron-transport and light-harvesting block (the structures of the block copolymers are
shown in Figure 1-26). The polymers exhibit nanostructured bulk heterojunctions (BHJ)
required for photovoltaic applications when casting into thin films. The nanodomains are
15 nm in diameter, which is in the same range as the exciton diffusion.
57
Figure 1-26. Various amorphous-crystalline donor–acceptor block copolymers synthesized by NMP. Adapted with permission from ref. 38 and 39. Copyright 2010 The Royal Society of Chemistry, and 2007 John Wiley & Sons.
Figure 1-27. Polymerization of donor-acceptor block copolymers via NMP. Adapted with permission from ref. 39. Copyright 2007 John Wiley & Sons.
The synthesis of the block copolymers are shown in Figure 1-28. The donor
block, PvTPA, was prepared using the common unimolecular alkoxyamine, N-tert-Butyl-
N-(2-methyl-1-phenyl propyl)-O-(1-phenyl ethyl)hydroxylamine, as the initiator and 10
mol% to 50 mol% of TIPNO free nitroxide were added to obtain low polydispersity. The
resulting macroinitiator (26, 27, 28) had PDIs from 1.14 to 1.26. The second block,
PPerAcr, was initiated from the macroinitiators. In order to achieve long block of
PPerAcr, the polymerizations were undertaken in concentrated PerAcr in the absence of
free nitroxide. However, 5 mol% of styrene was added in the system to maintain
sufficient control. The resulting block copolymers had ~80 wt% of PPerAcr block and the
PDIs were controlled to be from 1.14 to 1.26. These results show that NMP is capable
of preparing block copolymers even with monomers having rigid side groups.39
As for the G2-b-PPerAcr copolymer, it was synthesized from the dendronized
alkoxyamine initiator, G2-TIPNO.93 Starting from the functionalization of the commercial
available alkoxyamine, TIPNO-BzCl, G2-TIPNO was prepared via nucleophilic
displacement of chloride under basic conditions to form ether linkage with G2 dendron
containing TPA units. The resulting dendronized initiator controlled the polymerization of
PerAcr monomer, obtaining polymers with different molecular weights but all with PDIs
around 1.1 (Figure 1-29). This example demonstrated the possibility of alkoxyamine
initiator functionalization and preparation of polymers with functionalized end-group via
NMP.
59
Figure 1-28. Synthesis of dendronized initiator and subsequent polymer prepared via NMP. Adapted with permission from ref. 93. Copyright 2009 John Wiley & Sons.
Fréchet et al.9 also employed NMP to prepare block copolymers and applied
them in white emitting LEDs. The block polymer, (TPA-r-Blue)-b-(OXA-r-Red), contains
two blocks of random copolymers. The first copolymer block includes blue-emitting
Ir(dfppy)2(tpzs) and hole-transporting triphenylamine (TPA) moieties, while the second
block consists of red-emitting Ir(pq)2(tpzs) and hole-transporting oxadiazole (OXA)
moieties. By controlling the molecular weight of the polymer and relative content of two
blocks, phase separation of the blue and red blocks can be tuned to make the
nanostructured domain size large enough to achieve the isolation of the two
60
phosphorescent emitters and suppress energy transfer between the two emitters.
During the polymerization, the “blue” block was prepared first, resulting the
macroinitiator 36. The “red” block was initialized by the macroinitiator subsequently. The
final block copolymers had PDIs varying from 1.2 to 1.5. The phase separation domain
sizes were controlled to be from 26 nm to 49 nm by the structures of block copolymer,
which was larger than the exciton diffusion length and surpressed the energy transfer
between two iridium complexes.
Figure 1-29. Block copolymer prepared via NMP for LED application. Adapted with permission from ref. 9. Copyright 2006 John Wiley & Sons.
1.3.3.3 Reversible addition-fragmentation chain transfer (RAFT) polymerization
While ATRP and NMP are controlled radical polymerizations based on
reversible termination mechanisms, RAFT is a CRP technique based on the reversible
chain-transfer mechanism.69-70 The key feature of the mechanism of RAFT
polymerization is a sequence of addition-fragmentation equilibriums as shown in Figure
1-30. The initiation occurs as in conventional free radical polymerization and
61
conventional initiator such as AIBN and BOP are utilized. In the early stages of the
polymerization, addition of a propagating radical (Pn•) to the thiocarbonylthio compound
(RSC(Z)=S, 38, referred as RAFT agent or chain transfer agent (CTA) produces the
intermediate radical (39), which is followed by fragmentation and provides a polymeric
thiocarbonylthio compound (PnS(Z)C=S, 40) and a new radical (R•). This radical (R•)
reacts with monomer and forms a new propagating radical (Pm•). Rapid equilibrium
between the active propagating radicals (Pn• and Pm•) and the dormant polymeric
thiocarbonylthio compounds (40) provides equal probability for all chains to grow and
allows for the production of polymers with narrow polydispersity. When the
polymerization is complete (or stopped), most chains retain the thiocarbonylthio end
group and can be isolated as stable materials. The polymer with thiocarbonylthio end
group can act as macro chain transfer agent and is able to control the polymerization of
the second monomer to prepare another block.94
62
Figure 1-30. Mechanism of RAFT polymerization. Reproduced from ref. 94. Copyright 2006 CSIRO.
RAFT polymerization has the same condition as conventional free radical
polymerization with only the addition of thiocarbonylthio RAFT agents to induce rapid
addition-fragmentation transfer. A wide range of thiocarbonylthio compounds can be
used as RAFT agents, including certain trithiocabonates, diithioesters, xanthates,
dithiocarbanmates and other compounds. The structure features of RAFT agents are
shown in Figure 1-31 and examples of commonly used RAFT agents are presented in
Figure 1-32. The effectiveness of the RAFT agents depends strongly on monomers and
the properties of the free-radical leaving group R and the group Z which can be chosen
to activate or deactivate the thiocabonyl double bond and modify the stability of the
intermediate radicals. Tuning structures of R and Z groups allows introducing functional
63
groups into the RAFT reagents, thus making end-group post-modification of resulting
polymer possible.94
Figure 1-31. Structural features of RAFT agents. Reproduced from ref. 94. Copyright 2006 CSIRO.
Figure 1-32. Examples of RAFT agents. Adapted from ref. 95. Copyright 2011 The Royal Society of Chemistry.
RAFT has the most tolerance over monomers among the three CRP processes;
in principle, all monomers that can be polymerized by conventional free radical
polymerization can also be polymerized by RAFT polymerization in the presence of
efficient chain transfer reagents.36 Other advantages of RAFT over ATRP and NMP
include no metal catalyst involving and normal free radical polymerization conditions
employing (high reaction temperature is not required).95 One disadvantage of RAFT is
that polymers obtained by RAFT polymerization have dithioester groups, which have
64
some associated odors and colors.65 The sulfur-containing end-groups may have side
effects on the performance when the polymers via RAFT polymerization are utilized in
optoelectronic applications. Dithioester groups are known to be efficient fluorescence
quenchers.96 Even though dithioester groups can be easily hydrolyzed to thiol group, it
can also quench excited states of pendant chromophores.97 However, RAFT
polymerization has still been employed in the preparation of functional polymers for
optoelectronic applications.95
Figure 1-33. Preparation of light-harvesting polymers via RAFT polymerization. Adapted with permission from ref. 36. Copyright 2010 The Royal Society of Chemistry.
Ghiggino and Thang et al.36 prepared a light-harvesting polymer by RAFT
polymerization containing acenaphthyl energy donors and a terminal anthryl energy
acceptor (P(AcN)-AN, 42). The homopolymer has a low PDI of 1.08. Because of the
energy transfer from acenaphthyl to dithioester, the energy transfer efficiency from
acenaphthyl to anthryl is only 15%. This value can be increased up to 70% by using
P(AcN)-AN as a macroCTA to install a poly(methyl acrylate) block to separate
dithioester group from the poly-acenaphthyl block. The synthesis of the polymers is
illustrated in Figure 1-33.
65
Many vinyl monomers with pendant functionalities were polymerized by RAFT
polymerization, either by homopolymerization or copolymerization with other monomers
such as styrene and methyl acrylate, and were employed in the optoelectronic
application. These monomers include organometallic compounds, carbozole and
triphenylamine derivatives, arylene diimides and boron-cantaining monomers. The
structures of examples of these functional monomers are presented in Figure 1-34.
66
Figure 1-34. Monomers with pendant functionality polymerized by RAFT and used in optoelectronic applications. Adapted from ref. 95. Copyright 2011 The Royal Society of Chemistry.
67
1.3.4 Metathesis Polymerization
Metathesis polymerization is based on olefin metathesis reaction, in which two
carbon-carbon double bonds are reacted to form two new olefins, following Chauvin’s
exchange mechanism (as shown in Figure 1-35).98-99 Initially reported in 1950s, olefin
metathesis has been widely used from small molecular synthesis to polymer
preparation. Olefin metathesis can be used to form complex cyclic systems and medium
and large rings difficult to be achieved previously, which is particularly useful in
pharmaceutical chemistry.98-99 When cyclic olefins or acyclic dienes are involved,
polymers are formed in the metathesis reactions, referred as ring-opening metathesis
polymerization (ROMP) and acyclic diene metathesis (ADMET), respectively.
Figure 1-35. Chauvin mechanism of olefin metathesis. Reproduced with permission from ref. 99. Copyright 2005 Springer.
Figure 1-36. Types of olefin metathesis reactions. Reproduced with permission from ref. 99. Copyright 2005 Springer.
cyclooctenes, cyclooctadienes, cyclooctatetraenes, and many others. For advanced
functional polymer preparation, norbornene derivatives are doubtless the preferred
monomers.98, 100-101
ROMP has been widely used in the preparation of electro-active polymers with
pendant chromophores. Schrock and Rubner et al.102 synthesized electroluminescent
polymer by ROMP in 1997. The blue-emitting norbornene derivative was polymerized
via ROMP by Schrock-type [Mo] catalyst; the polymerization had a 95% conversion
yields and produced a polymer with a low PDI of 1.04 (as illustrated in Figure 1-40). The
same strategy was also applied to the polymerization of electron-transporting and hole-
transporting norbornene derivatives, in which polymers with low PDI values of 1.02-1.08
were obtained.
Figure 1-39. ROMP of a blue-emitting polymer with Mo catalyst. Adapted with permission from ref. 102. Copyright 2009 American Chemical Society
An advantage of ROMP is its tolerance of metal-containing monomers when
using Grubbs’ ruthenium catalysts. Sleiman et al.103-104 prepared polymers with pendant
ruthenium tris(bypridine) (Ru(bpy)3) complexes (44, Figure 1-41). The
homopolymerzation of 44 finished in 20 minutes with the third generation Grubbs’
catalyst (45). The monomer conversion was observed to be linear depended on reaction
time.103 Block copolymer containing an alkyl substituted group (47) were also prepared.
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By varying the ratio of alkyl block and Ru block, the block copolymer self-assembled
into different shapes such as vesicles, tubes, bowls and stars.104 Other cyclic olefin
monomers containing ruthenium,105 iridium,106-107 platinum108-109 and aluminum110
complexes have also been polymerized via ROMP.
Figure 1-40. Preparation of polymers with pendant Ru complexes by ROMP. Adapted with permission from ref. 103 and 104. Copyright 2004 and 2007 American Chemical Society.
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Figure 1-41. Examples of polymers with pendant metal-complexes prepared via
ROMP.105-110
1.3.4.2 Acyclic diene metathesis (ADMET)
ADMET polymerization is performed on α, ω-dienes to produce strictly linear
polymers with unsaturated polyethylene backbone. While ROMP is chain-growth
polymerization, ADMET is step-growth polymerization, which is a thermally neutral
process and driven by the release of ethylene. Based on the nature of step-growth
polymerization, ADMET is not “living polymerization”. However, ADMET creates perfect
regionregular polyethylenes with precisely placed braches, because of near-quantitative
monomer to polymer conversion and few side reactions.99 Due to the perfectly
regionregularity, ADMET is noteworthy in the side-chain polymer preparation. An
example of electroactive polymers prepared via ADMET was reported by Reynolds and
Wagener et al.11-12 The parent polymer with boron esters was prepared by ADMET
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polymerization first, and then blue, green and red-emitting chromophores were attached
to the polymer via Suzuki coupling reactions.
Figure 1-42. Preparation of electroactive polymers via ADMET and post-functionalization. Adapted with permission from ref. 12. Copyright 2010 American Chemical Society.
1.4 Preparation of Side-Chain Conjugated Polymer: Post-Polymerization
Modification
With the development of living polymerization, it allows for the synthesis of
functional polymers with precisely defined molecular weight, composition and
architecture. Direct polymerization of functional monomers is apparently a more
attractive strategy; however, there are still disadvantages of this strategy. The first one
is monomer tolerance. Although there is significant improvement over tolerance of
functional groups in monomers in CRP and ROMP, there is still a broad range of
monomers with side-chain functionalities that cannot be directly polymerized using any
currently available controlled polymerization techniques. Such functional groups may
either completely prevent controlled polymerization or participate in side reactions that
lead to loss of control over the polymerization. And in some cases, the large monomer
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will introduce steric hindrance so that the molecular weight of resulting polymer is too
low.111 The second is economic consideration of monomers with functional groups
synthesis and purification. Generally living polymerizations such as CRP will lose
control if the conversion of monomer is high. Some monomers, which take a lot of time
and effort to prepare, will waste during the polymerization. This is economically
undesirable, especially for functional groups with rare-earth metals. On the other hand,
because of the high purity requirement for monomers in the living polymerization, the
cost of purification of synthetic monomers may be very high.112
Post-polymerization modification, also known as polymer-analogous
modification,112-113 is an alternate strategy that can overcome the technical and
economical limitation of direct polymerization of functional monomers. As illustrated in
Figure 1-43, post-polymerization modification is based polymerization of monomers with
functional groups that are inert towards the polymerization conditions, but can be
quantitatively converted in a subsequent reaction step into a broad range of other
functional groups.112 As the functional monomers to be polymerized are inert to the
corresponding polymerization, the monomer tolerance limitation has been overcome. In
addition, the post-polymerization modification strategy is also highly attractive for
combinatorial materials discovery. When a single reactive polymer precursor is
prepared, a diverse library of functional polymers with identical chain lengths and chain-
length distributions can be generated based upon the parent precursor. With orthogonal
modification reactions, copolymers with different functional groups can also be
prepared.112 In this section, different reactive polymer precursors and post-
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polymerization modification reactions used in the preparation of side-chain conjugated
polymers will be briefly reviewed.
Figure 1-43. Synthesis of polymers by post-polymerization modifications. Reprinted with
permission from ref. 112. Copyright 2006 John Wiley & Sons.
1.4.1 SN2 Reaction
SN2 nucleophilic displacement of a leaving group in the polymer precursor might
be the easiest post-polymerization modification reaction. Meyer et al.8, 114-118 prepared a
variety of functional polymers stating from styrene-p-(chloromethyl)styrene copolymers,
in which chloride is a good leaving group. Via SN2 reaction with bypridine, amine,
alcohol and acid, conjugated functional groups were attached to the parent copolymers
with C-N, ether and ester linkages, as illustrated in Figure 1-44. However, the SN2
reactions with these nucleophiles are not quantitative.
76
Figure 1-44. Post-polymerization modification via SN2 reactions.8, 114-118
1.4.2 Amide Coupling
Papanikolas and Meyer et al.53-54, 58, 119 also reported the preparation of amide-
linked polypyridylruthenium- deviated polystyrenes from the amide coupling reaction
between amine-containing polystyrenes and carboxylic acid-bearing
polypyridylruthenium complexes. The parent polymer precursors were prepared by
conventional free radical polymerization58, 119 and living anionic polymerization.53-54 The
reaction condition was “borrowed” from peptide chemistry, in which the amide coupling
reaction between primary amine and carboxylic acid was catalyzed by the combination
of 4-(dimethylamino)pyridine (DMAP), 4-methylmorpholine (NMM), benzotriazol-1-yloxy
tris(dimethylamino)phosphonium hexafluorophosphate (BOP) and 1-hydroxybenzo
triazole hydrate (HOBT), as presented in Figure 1-45. The coupling reaction was
quantitative according to 1H NMR characterization.53, 119
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Figure 1-45. Post-polymerization modification via amide coupling.53-54, 58, 119
1.4.3 Reaction with Active Ester
Since the pioneered work by Ferrti and Ringsdorf et al.,120-121 a broad variety of
active esters have been developed, introduced into vinyl monomers and polymerized by
different living polymerization techniques. The commonly used active ester includes N-
thiazolidine-2-thione (TT), acryloyl oxime (AO) and nitrophenyl (NP) groups, in which
NHS and PFP are most frequently employed.113 The structures of these active esters
are shown in Figure 1-46. On the other hand, the vinyl monomers of active esters are
normally based on methacrylates (MAs) and 4-vinyl benzoate (VB).
Figure 1-46. Structures of active esters.113
The reaction between active ester polymers with amines is the most frequently
used in the post-polymerization modification to react with active esters and make amide
linkage. Because of their good nucleophilicity, amines are able to react selectively even
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in the presence of other weaker nucleophiles, such as alcohols.112-113 Because of the
high reactivity of active esters, the conversion from active ester to amide linkage is
normally quantitative. An example of employing active ester polymer as the precursor to
prepare side-chain conjugated polymer was reported by Tew et al.;122 N-succinimide p-
vinyl benzoate (NHSVB) was polymerized by RAFT, and then the resulting polymer was
reacted with an amine functionalized terpyridine (terpy), creating a new homopolymer
containing terpy ligands on every monomer, as demonstrated in Figure 1-47. The
pendant terpy group can be further utilized for metal-coordination to install metal
complexes onto the polymer.
Figure 1-47. Post-polymerization modification via the reaction between amine and active ester. Adapted with permission from ref. 122. Copyright 2006 John Wiley & Sons.
1.4.4 Metal-Coordination Reaction
As discussed in the previous section, one way to prepare metallopolymers with
metal-containing pendants is to directly polymerize monomers with pendant metal
complexes.95 However, the common way to prepare this type of metallopolymers is to
prepare polymer precursors with metal-coordination (also referred as metal-ligation)
functionalities and introduce metal species by post-functionalization. The most
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frequently employed metal-coordination functional groups are terpyridine (terpy,
coordinates with ruthenium)123 and acetoacetate (acac, coordinates with platinum10 and
iridium124). Examples of the reaction are presented in Figure 1-48. The yield of metal-
coordination in the polymeric system can hardly reach 100%. It is reported that the
coordination between terpyridine and ruthenium was 75%, according to elemental
analysis;123 and the functionalization of acac with iridium was around 88% from 1H NMR
characterization.124
Figure 1-48. Examples of metal-coordination reactions in post-polymer modification.10,
123-124
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1.4.5 Palladium-Catalyzed Coupling and Cross-Coupling Reactions
The Palladium-catalyzed Heck, Sonogoshira, Suzuki, and Stille reaction produce
C-C bonds in high yield under relatively mild reaction conditions, Thus these reactions
have also been employed in post-polymerization modifications. Chan et al.125 reported
Heck reaction on the polystyrene-b-polyisoprene block copolymers, in which the
functionalization yield varied from 22% to 44%. Reichmanis et al.126-127 reported the
stannylation of polystyrene and subsequent Stille reaction. The yield of stannylation can
reach more than 96% and the final Stille coupling was reported to be quantitative.
Grubbs et al.128 have explored the optimization of reaction conditions for the
modification of poly(p-bromostyrene) with phenylacetylene and 1-hexyne by
Sonogashira coupling. With (PhCN)2PdCl2 as catalyst and Tri-tert-butylphosphine as
additional ligand, the conversion of bromide can reach up to 100% and 89% for
phenylacetylene and 1-hexyne. As for Suzuki coupling, the preparation of boron-
containing polymers has been achieved by ATRP129-131 and RAFT;132 however, there is
no report of Suzuki coupling based on these polymers yet.
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Figure 1-49. Palladium-catalyzed coupling reactions in post-polymerization modification.
1.4.6 “Click Chemistry”
Considered as “click reaction”, the copper(I)-catalyzed Huisegen 1, 3-dipolar
cycloaddition (also referred as Cu(I)-catalyzed azide-alkyne cycloaddition, CuAAC) has
been extensively employed in post-polymerization modification, because it gives high
yields under mild and simple reaction conditions in both aqueous and in organic media,
generating only inoffensive byproducts.133-136 The brief description of the mechanism of
CuAAC is illustrated in Figure 1-50.137
82
Figure 1-50. Copper(I)-catalyzed azide-alkyne cycloaddition. Reproduced with
permission from ref. 137. Copyright 2013 The American Association for the Advancement of Science.
The polymer precursors for CuAAC, bearing azide, alkyne, and protected alkyne
groups, have been polymerized by a variety of living polymerization techniques.112-113,
136 Certain functional groups, such as epoxy133, 138 and benzyl halides,97 can be
converted to azide groups quantitatively. Figure 1-51 presents examples of post-
polymerization modification using CuAAC click reaction.97, 139
83
Figure 1-51. Application of CuAAC click reaction in post-polymerization modification.97,
139
Besides CuAAC click reaction, there are several other resections belonging to
“click chemistry”, such as Cu-free azide-alkyne cycloaddition, thiol-ene, thiol-yne, Diels-
Alder, thiol-bromo, etc. It is possible to emerge different “clickable” functional groups
into one polymer chain, and utilize different click reactions subsequently and
orthogonally.140
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Figure 1-52. Preparation of copolymers with different functional groups with orthogonal
click reactions. Reprinted with permission from ref. 139. Copyright 2009 American Chemical Society.
1.5 Scope of Present Study
In this dissertation, the author intends to utilize the post-polymerization
modification strategy to prepare several families of side-chain conjugated polymers for
fundamental photophysical studies and materials applications. The goal for the polymer
preparation is to synthesize polymers with desired molecular weight and low
polydispersity, and quantitative chromophores loading during the post-polymerization
modification process. To this end, a synthetic strategy is designed, as demonstrated in
Figure 1-53. The polymerizations started with commercial available monomers, glycol
methacrylate (GMA) and 4-vinylbenzyl chloride (VBC), because they are economically
desirable and easy to purify. The polymerization techniques employed were RAFT and
NMP. The epoxy group in GMA and chloride in VBC can be converted to azide group
quantitatively in mild conditions via SN2 reactions. Finally, organic and organometallic
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chromophores bearing a terminal alkyne group were grafted onto the azide-containing
clickable polymer precursors.
Figure 1-53. Synthetic strategy of side-chain conjugated polymers in this dissertation.
Several families of side-chain conjugated polymers were prepared employing this
synthetic strategy. First of all, a polymer bearing pendant non-linear absorption
chromophores (poly-FBPt) was prepared. The resulting polymer conserved all the
photophysical properties of the small molecular chromophore, including the non-linear
absorption property. The polymer can be cast into films easily and the polymer film still
showed triplet-triplet absorption and non-linear absorption. The results of this work
provide insight regarding the introduction of platinum acetylides into polymers for optical
applications. Secondly, different chromophores were installed onto polystyrene-based
backbone, acting as energy donor and acceptor, respectively. The singlet and triplet
energy transfer was characterized and studied in polymeric systems with organic and
organometallic chromophores. Finally, arrays of polypyridine ruthenium(II) complexes
with polystyrene backbone were prepared and employed in dye-sensitized solar cells
(DSSCs).
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CHAPTER 2 NONLINEAR ABSORPTION POLYMERIC ARRAY FROM CONTROLLED RADICAL
POLYMERIZATION AND “CLICK” CHEMISTRY
2.1 Background
Nonlinear absorption (NLA) refers to the change in transmittance of a material as
a function of light intensity or fluence. At sufficiently high intensities, the probability of a
material absorbing more than one photon before relaxing to the ground state can be
greatly enhanced.141 So an optical device based on NLA materials exhibits a linear
transmittance below a specific input intensity or fluence level but, above this level, its
output intensity varies non-linearly with input. NLA optical materials play a major role in
the photonic technologies, and have been used in several applications, such as passive
mode locking, pulse compression, and the most popular application: eye and sensor
protection in optical systems (e.g., telescopes and night vision systems).142 There are
mainly two mechanisms for nonlinear absorption, including two-photon absorption (TPA)
and excited-state absorption (ESA).
Two-photon absorption is the simultaneous absorption of two photons resulting in
the excitation of a chromophore from the ground state to a higher-lying state, as
demonstrated in Figure 2-1a.141, 143 This process involves different selection rules from
those of single-photon absorption. TPA strength of a chromophore depends on the two-
photon absorption cross section (σ2). General guidelines of designing a TPA
chromophore include (1) extended conjugation length with planar chromophores; (2)
motifs such as D-π-D, A-π-A, and D-π-A (where D = donor, A = acceptor and π =
conjugated spacer); (3) increasing the π-donor or π-acceptor strength; (4) introduction
of more polarizable unsaturated bonds; and (5) variation in the nature of the conjugated
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bridge.143-145 TPA chromophores properties can be installed into organic molecules,
liquid crystals, conjugated polymers, fullerenes, coordination and organometallic
compounds, porphyrins and metalloporphyrins, nanoparticles, and biomolecules and
derivatives.143, 146
Figure 2-1. Mechanisms of nonlinear absorption. A) Jablonski diagram for a typical chromophore that exhibits two photon absorption, intersystem crossing, and
triplet-triplet absorption. B) Schematic illustrating idealized ground state and
excited state difference absorption spectra for a chromophore that exhibits reverse saturable absorption. Adapted with permission from ref. 142. Copyright 2011 American Chemical Society.
In addition to two photon absorption, reverse saturable absorption (RSA) is
another pathway for nonlinear absorption. RSA chromophores feature a small but non-
zero ground state absorption cross-section (σ1) and a large excited state absorption
(ESA) cross-section (σESA) at the same wavelength region of the ground state
absorption (Figure 2-1b).143, 145 The ESA originates mostly from the lowest-energy
singlet or triplet (S1 or T1), but can be originated from all possible transient states.
Generally, as the light intensity increases, the population of the singlet and triplet
excited states increases. In RSA, the excited state exhibits more absorption than the
ground state, resulting in more absorption as the incident optical flux increases. The
magnitude of RSA is determined by the ratio σESA / σ1 determines; the larger the ratio,
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the stronger the RSA. RSA chromophores includes porphyrins, indanthrones, metal
cluster compounds, fullerenes, cyanines, phthalocyanines, and naphthalocyanines.143
It is possible to combine TPA and ESA mechanisms for the nonlinear absorption,
and platinum acetylides are suitable candidate chromophores. Typical platinum
acetylide complexes feature a four-coordinated square planar platinum(II) center with
the general formula PtL2(CΞCR)2, where L is typically a phosphine ligand (e.g., PBu3)
and CΞCR is an aryl acetylide unit (i.e., R = aryl).147 When R is a TPA chromophore,
such as benzothiazole-2,7-fluorene (FB) and diphenylamino-2,7-fluorene (DPAF), both
simultaneous TPA and one-photon absorption can populate the singlet excited state
(S0→S1). As a heavy metal, platinum can induce singlet to triplet (S1→T1) intersystem
crossing (ISC) via spin-orbit coupling with a rapid dynamics and high efficiency (ΦISC >
90%). The populated T1 state will result in the strongly allowed T1→Tn transitions (ESA
pathway) in the time of 10 to 400 ps time scale. The combination of TPA and ESA will
give strong overall nonlinear absorption in fs to µs scale, because TPA occurs
instantaneously (fs to ps), and then the triplet state is produced by ISC and persists into
the ns to μs time scale. Nonlinear absorption via TPA/ESA is most efficient in
chromophores where the 2PA absorption maximum coincides spectrally with ESA.143
Some examples of platinum acetylide complexes exhibiting TPA/ESA nonlinear
absorption properties are presented in Figure 2-2.148-149
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Figure 2-2. Examples of platinum acetylides with TPA/ESA mechanisms.148-149
To utilize platinum acetylides for practical NLA materials, it is necessary to
incorporate into polymeric materials. Pt acetylides can be doped into poly(methyl
methacrylate) (PMMA) matrix to obtain a guest-host solid. Pt acetylides with multi-
acrylate functionality (Figure 2-3A) can also be cross-linked with MMA monomers. Sol-
gel processes are also employed to form glassy materials (Figure 2-3B).
Figure 2-3. Platinum acetylides used in NLA materials.
Efforts have also been made to prepare linear polymers containing platinum
acetylide units. Our group has prepared copolymers with platinum acetylide-containing
acrylate monomers and methyl methacrylate (MMA) monomers via free radical
polymerization. However, it was found the maximum loading of platinum acetylide
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chromophores can only reach 12%.111 In order to obtain polymers with high NLA
chromophore loading, a post-polymerization modification strategy was utilized. A
“clickable” polymer precursor was constructed first, and the nonlinear absorption
platinum acetylide chromophores were installed onto the polymer backbone via
quantitative-yield copper(I)-catalyzed azide-alkyne cycloaddition (“click” reaction). The
resulting polymer has chromophores in each repeat; it has typical polymer properties,
such as film-forming ability and processing advantages, and NLA properties conserved
from small molecular chromophores.
2.2 Polymer Design and Preparation
The well-defined nonlinear absorption polymeric array prepared in this chaptor
features a flexible, atactic and non-conjugated polyacrylate backbone and pendent
small molecular NLA platinum acetylide chromophores. The structure of the NLA
chromophore (FBPt) and the polymer (Poly-FBPt) are shown in Figure 2-4.
Figure 2-4. Structures of FBPt and Poly-FBPt.
2.2.1 Synthesis of FBPt
The structure of the NLA chromophore (FBPt) combines a fluorenebenzothiazolyl
(FB) unit and platinum(II) metal center. FB is a well-known D-π-D motif, yielding
materials that have significant near-IR TPA cross-section. Platinum is used to induce
rapid intersystem crossing from singlet to triplet with high yields. As stated in the
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previous section, FBPt has the nonlinear absorption property with a combination of TPA
and ESA pathways. 150
Figure 2-5. Synthetic route of FBPt.
The synthetic route of FBPt is shown in Figure 2-5. Starting from the
trimethylsilyl-protected benzothiazolylfluorene (FB), deprotection of compound 1
resulted in compound 2 with a terminal alkyne. Efficient synthesis of monoalkynyl
platinum(II) complex (3) was synthesized through reaction between compound 2 and
cis-Pt(PPh3)2Cl2. The Hagihara coupling in the absence of copper(I) species allowed
mono-substitution of a chloride of cis-Pt(PPh3)2Cl2 with alkyne. Subsequent coupling
between compound 3 and the mono-protected diethynylbenzene (4) generated the
asymmetric dialkynyl Pt(II) complex (5) under mild conditions with a catalytic amount of
copper(I) iodide. Deprotection of 5 with MnO2 and KOH resulted in the FBPt
chromophore, which had a terminal alkyne and was ready for the Cu(I) catalyzed azide-
alkyne cycloaddition (“click” reaction).
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2.2.2 Preparation of polymer backbone and poly-FBPt
The route used to synthesize Poly-FBPt is outlined in Figure 2-6, which started
from a commercially available monomer, GMA. It was with low cost and easy to purify.
The epoxy ring in GMA can be opened with nucleophile azide salts (such as sodium
azide) to make it “clickable”. The narrow molecular weight distribution poly(glycal
methacrylate) PGMA (7) was polymerized via reversible addition-fragmentation transfer
(RAFT) polymerization of monomer, with 2-cyanoprop-2-yl-1-dithionaphthalate (CPDN,
6)151 as chain transfer agent (CTA).
The structure of the chain transfer agent, CPDN, involves a isobutylnitrile group
(R), a good free radical leaving group both in absolute terms and relative to the
propagating species derived from the monomer being polymerized, and a naphthyl
group as Z group, which enhance the reactivity of the C=S double bond. The synthesis
of CPDN was a “one-pot” reaction which started from Grignard reaction 1-
bromonaphane and carbon disulfide. The resulting Grignard reagent was oxidized by
DMSO, and then the disulfide bond was cleaved via free radical reaction with AIBN to
give the chain transfer agent, CPDN, as illustrated in Figure 2-6.151
GMA was polymerized in the presence of both CPDN and AIBN (4:1 molar ratio).
The degree of polymerization (DP) of the resulting PGMA can be estimated from its
molecular weight, which was 42. The pendent epoxy groups were ring-opened with
excess NaN3 in the presence of NH4Cl, affording poly(hydroxyazidopropyl methacrylate)
(PHAZPMA, 8) bearing one hydroxyl group and one azido group in each repeat unit.152
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Figure 2-6. Synthesis of Poly-FBPt.
GPC analysis of 8 revealed a monomodal and symmetric peak, indicating there
was no cross-linking or branching during the ring-opening reaction with NaN3 (Figure 2-
7). The ring-opening reaction can also be evidenced by the 1H NMR spectra (Figure 2-
8). After reaction, resonance signals characteristic of epoxy moieties, 2.61 and 2.75
ppm (-CH2O in the epoxy group), 3.16 ppm (-CHO in the epoxy group) and 3.68 and
4.27 ppm (-OCH2CH- next to the epoxy group), completely disappeared. On the other
hand, the arising of new peaks, 3.35 ppm (-CH2N3), 3.87 ppm (-OCH2CHOH) and 5.49
ppm (-CHOH), further confirmed the consumption of the epoxy group in the ring-
opening reaction with NaN3. The FTIR spectra (Figure 2-10) can also evidence the ring-
opening transformation. After reaction, the absorption peak at 909 cm-1, which was
corresponding to the epoxy group, disappeared. And on the other hand, a strong band
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at 2100 cm-1 was present in PHAZPMA, which is the absorption band of the azido
moieties.
The dithioester group is believed to be removed affording a thiol end-group.52, 153
Although it is difficult to prove it from 1H NMR spectra, it can be evidenced by UV-vis
absorption. The dithioester absorption peak in PGMA, which is at 310 nm, disappeared
after treating with sodium azide, as shown in Figure 2-9. The color change of polymers,
from pink (PGMA) to white (PHAZPMA), is caused by the dithioester end-group
removal. The 282 nm peak in the spectrum of PHAZPMA is due to absorption of azide
a Ground state absorption maxima b Fluorescence spectra, obtained by excitation at ground state absorption maxima c With anthracene as quantum yield standard, ϕ = 0.27 in ethanol d Triplet excited state lifetimes, from time-correlated single photon counting (TCSPC), and ⟨τ⟩ is median lifetime calculated as ⟨τ⟩ = ΣAiτi.
e Phosphorescence spectra, obtained by excitation at ground state absorption maxima f Triplet-triplet transient absorption maxima
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2.3.2 Triplet-triplet Transient Absorption
Figure 2-13. Transient absorption spectra of FBPt (blue) and Poly-FBPt (red) in deoxygenated THF. Excited at 355 nm.
The triplet excited state is further studied via triplet-triplet transient absorption
(TA), as presented in Figure 2-13. Near-UV excitation at 355 nm generates strongly
absorbing transients for the small molecular platinum acetylide FBPt and the NLA
polymer Poly-FBPt in THF. The shapes of TA spectra for FBPt and Poly-FBPt are
similar; both show negative bands from 350 - 400 nm corresponding to bleaching of the
ground state absorption and positive triplet-triplet (T1-Tn) excited state absorption bands
102
across most of the visible region, with maxima at 670 and 675 nm. The polymer has a
slightly red-shift in the transient absorption.
On the other hand, the triplet excited state lifetime of Poly-FBPt is much smaller
than that of FBPt (5.45 µs vs. 336 µs). This result agrees with the significant decrease
of phosphorescence quantum yields, and further confirms the chromophore quenching
interaction for Poly-FBPt. However, the existing of triplet-triplet (T1-Tn) excited state
absorption of the polymer suggests it can still be a candidate NLA material.
2.3.3 Nonlinear Absorption Response
All ground state absorption, steady-state photoluminescence and triplet-triplet
transient absorption have suggested that the resulting polymer, Poly-FBPt, exhibits
similar photophysical characteristics as the small molecular NLA chromophore FBPt; in
addition, NLA response of the Poly-FBPt polymer were examined by nanosecond open-
aperture z-scan.143
Figure 2-14. Schematic diagram of a simple open-aperture z-scan apparatus. Adapted
with permission from ref. 142. Copyright 2009 American Chemical Society.
Open-aperture z-scan technique is a nonlinear transmission method, and a
typical setup of a simple open-aperture z-scan apparatus is illustrated in Figure 2-14. A
beam splitter (BS) is utilized to divide the single laser beam into two equal parts; one
will be detected by Detector 1 as the reference and the other will be focused by the
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plano convex lens, pass through the sample and finally collected Detector 2. The signal
strength ratio by Detectors 2 and 1 (I2/I1) can be considered as the nonlinear absorption
response. The sample is placed in the light path between convex lens and Detector 2,
and it is able to move one-directional along the light path (z-axis), in which the light flux
intensity is different at different positions. When the sample is placed at the focal point
of the convex lens (z = 0), it receives highest light intensity; while at positions other than
the focal point, the light intensities are lower; the farther away from z = 0 position, the
lower the light intensity. In summary a “V-shape” curve of I2/I1 vs. z will be obtained for a
NLA material, and I2/I1 should has the lowest value when z = 0.111, 143
NLA measurements of FBPt and Poly-FBPt were conducted using 1 mM
solutions in benzene, and the known DPAF-capped diplatinum acetylide, T2 (Figure 2-
15), was used as the benchmark.143 The solution concentration of poly-FBPt was based
on the platinum acetylide-containing repeat unit. An excitation wavelength of 600 nm
was selected due to the lack of appreciable ground state absorption at this wavelength.
The Poly-FBPt solution clearly displays ~ 7% attenuation of the transmittance at z = 0
position, which is nearly identical to FBPt. This observation strong evident that Poly-
FBPt conserved the non-linear absorption property from FBPt.
104
Figure 2-15. NLA response of 1 mM solutions of blank (black square), FBPt (blue triangle), Poly-FBPt (red inverted triangle) and T2 (pink circle)
Figure 2-16. Structure of z-scan benchmark, T2.
2.4 Photophysical Characterization of Thin Film
A major goal of designing the side-chain conjugated polymer is to utilize the film-
forming property of polymers. Poly-FBPt was spin-coated to thin film and characterized
via UV-visible absorption, steady-state photoluminescence and triplet-triplet transient
absorption, and the photophysical parameters of the thin film are summarized in Table
2-1. The pictures of Poly-FBPt solution and thin film under visible light and UV light
(365 nm) are presented in Figure 2-17. THF solution of Poly-FBPt is light-green, and
105
exhibits week fluorescence under UV radiation. The Poly-FBPt thin film from spin-
coating is transparent and colorless.
Figure 2-17. Photos of Poly-FBPt solution and film under visible and UV light. A) Poly-FBPt in THF solution under visible light. B) Poly-FBPt in THF solution under UV light. C) Poly-FBPt film under visible light. D) Poly-FBPt film under UV light.
Figure 2-18. Ground state absorption and photoluminance of Poly-FBPt thin film.
106
Figure 2-18 displays ground state absorption and photoluminescence of Poly-
FBPt thin film. The absorption of the thin film is nearly identical to that of solution, with
an absorption maximum at 375 nm. The film has a ~20 nm red shift for fluorescence
comparing to the polymer solution. The red shift may due to the chromophore
interactions in the polymer thin film. However, the film has the same phosphorescence
maximum as the solution. The lifetimes of fluorescence and phosphorescence of the
polymer thin film exhibit higher value than those of polymer solution, but they are still in
the same magnitude.
Figure 2-19. Transient absorption of Poly-FBPt thin film
The transient absorption of Poly-FBPt thin film is recorded in Figure 2-19. The
positive bands, which present the triplet-triplet (T1-Tn) excited state absorption, are
similar to the solutions of FBPt and Poly-FBPt, displaying strong and broad absorption
across most of the visible region, with maximum at 675 nm.
Unfortunately, the setup of nanosecond open aperture z-scan was not optimized
for thin film samples. However, UV-visible absorption, steady-state photoluminescence
and triplet-triplet transient absorption have suggested the Poly-FBPt thin film have
107
similar photophysical characteristics as its solution, which exhibits non-linear absorption
property as the small molecular chromophore FBPt.
2.5 Summary
In this chapter, the design and preparation of polymer with nonlinear absorption
property were described. The NLA polymer, which features a flexible, non-conjugated
polyacrylate backbone and pendant platinum acetylide NLA chromophores, was
prepared via the RAFT-SN2-click strategy, starting from a commercial-available
monomer. The resulting polymer, Poly-FBPt, can be achieved with desired molecular
weight, low polydispersity index and high loading yields of NLA chromophores. The
photophysical properties of Poly-FBPt were characterized and the results showed that
the polymer conserved the properties of the small molecular chromophore FBPt,
displaying nonlinear absorption characteristics.
2.6 Experimental
2.6.1 Instrumentation and Methods
NMR spectra were measured on a Gemini-300 FT-NMR, a Mercury-300 FT-
NMR, or an Inova-500 FT-NMR. High resolution mass spectrometry was performed on
a Bruker APEX II 4.7 T Fourier Transform Ion Cyclone Resonance mass spectrometer
(Bruker Daltonics, Billerica, MA). FT-IR was measured in a Perkin-Elmer FTIR
spectroscopy. Gel permeation chromatography (GPC) analyses were carried out on a
system comprised of a Shimadzu LC-6D pump, Agilent mixed-D column and a
Shimadzu SPD-20A photodiode array (PDA) detector, with THF as eluent at 1 mL/min
flow rate. The system was calibrated against linear polystyrene standards in THF.
UV-visible absorption was carried out on a Shimadzu UV-1800 dual beam
absorption spectrophotometer using 1 cm quartz cells. Photoluminescence
108
measurements were obtained on a fluorometer from Photon Technology International
(PTI) using 1 cm quartz cells.
Luminescence lifetimes were obtained with a multichannel scaler/photon counter
system with a PicoQuant FluoTime 100 Compact Luminescence Lifetime
Spectrophotometer. A high-performance Coherent CUBE diode laser provided the
excitation at 375 nm (P < 10 mW). The laser was pulsed using an external Stanford
Research Systems DG535 digital decay and pulse generator with four independent
delay channels. At least four narrow band pass filters were used for measurements
followed by global fit processing (FluoroFit software). Decays were obtained using the
Figure 3-8. Steady-state absorption and emission of model compounds and polymers.
A) Steady-state absorption of model compounds 1a (green solid squares, ε375
nm = 58000 M-1cm-1) and 1b (red solid circles, ε470 nm = 24000 M-1cm-1). B)
Fluorescence emission of OPE (fl =1.0) and TBT (fl = 0.75). C) Steady-state absorption of polymers, P-0 (black hollow squares), P-1 (red hollow circles), P-5 (green hollow triangles), P-10 (blue hollow inverted triangles) and P-20 (brown hollow diamonds). All polymers were dissolved in THF. The inset is the absorbance at 470 nm for five polymers. D) Fluorescent spectra of polymers, P-0 to P-20. All polymers were dissolved in THF with an OD = 0.8 at 370 nm. The inset shows quantum yields of OPE emission (green solid squares, 370 nm to 510 nm) and TBT emission (red solid circles, 510 nm to 775 nm) for the five polymers. E) Fluorescence photographs of five polymers.
123
The absorption and fluorescence spectra of model compounds 1a and 1b are
shown in Figure 3-8A and 3-8B. OPE oligomer 1a features two near-UV absorption
bands (322 nm and 370 nm) and a single fluorescence band (407 nm) with a quantum
yield of unity, fl ~1.0. The TBT oligomer 1b, exhibits a near-UV absorption band at 330
nm and a visible band at 470 nm. The fluorescence band from 1b has a maximum at
~ 600 nm, with fl ~ 0.75.
The absorption spectra of the polychromophores are shown in Figure 3-8C. The
donor-only polymer, P-0, has essentially the same spectrum as that of OPE model
compound 1a. The spectra of P-1 to P-20 are dominated by the OPE based transitions
(especially at 370 nm); however, they show increasing TBT character as the loading of
the lower energy chromophores in the click grafting is increased (Figure 3-8C, inset).
This feature allows straightforward determination of the TBT loading in the co-polymer
from the ratio of the absorbance at 370 and 470 nm (see Table 3-1). The simulated
absorption spectrum of P-20 (Figure 3-9), calculated from the absorption spectra and
extinction coefficients of 1a and 1b, corresponds nicely with the measured P-20
absorption. Taken together, the absorption data for the model compounds and polymers
allow several conclusions. First, the fractional loading of OPE and TBT units in the
polymers closely corresponds to the stoichiometry used in the feed for the click
reactions. Second, the absorption at 370 nm is due almost exclusively to the OPE
(donor), making it possible to selectively excite this chromophore. Finally, the polymer
spectra are accurately simulated as a linear combination of the spectra of the OPE and
TBT chromophores, indicating that there is not a strong ground state interaction among
the individual units in the polymers.
124
Figure 3-8D shows the fluorescence spectra of all five polymers. In this
experiment the concentration of the polymer solutions was adjusted such that the
absorption at 370 nm excitation wavelength was identical. The fluorescence of the OPE
(donor) only polymer P-0 (λ = 413 nm) appears as a single band which is slightly red-
shifted and broadened compared to that of the OPE model 1a; this result suggests that
there is some interchromophore interaction in the singlet excited state. Interestingly, in
P-1, the emission from the OPE chromophore at 413 nm is quenched ~40% relative to
the intensity of P-0, and fluorescence from the TBT units at 600 nm is evident; indicating
that OPE to TBT energy transfer is efficient. Calculations based on the fluorescence
quantum yields of the donor and acceptor reveal that the energy transfer efficiency in P-
1 is ~55%. In P-5, the OPE emission is quenched to a greater extent, and the energy
transfer efficiency from OPE to TBT approaches 85%. This trend continues through the
series as seen in Figure 1d (inset) where the quantum yields of OPE and TBT
fluorescence in the P-x series is plotted. Note that the OPE emission yield decreases
sharply from P-0 to P-5, followed by a more gradual decline from P-10 to P-20. By
contrast, the TBT emission yield increases sharply from P-0 to P-5; however, it then
decreases slightly from P-5 to P-20. The latter trend is likely due to self-quenching of
the TBT chromophore as its concentration in the polymers increases, possibly due to
interchromophore charge transfer interaction between TBT units. The self-quenching
mechanism is supported by the fact that the lifetime of the TBT fluorescence also
decreases from P-5 to P-20 (see Table 3-1). The five polymers display different colors
upon UV radiation because of the different ratios of 413 nm and 600 nm emissions
(Figure 3-8E).
125
Figure 3-9. Comparison of measured (brown hollow diamonds) and calculated (cyan
hollow stars) absorption spectra of P-20. The calculated spectrum was derivate from the absorption spectra and extinction coefficients of OPE model compound 1a and TBT model compound 1b based on the equation ε(Calculated P-20) = 0.8 x ε(1a) + 0.2 x ε(1b). The results were then normalized to the spectrum (cyan hollow stars) above.
Additional evidence for OPE to TBT energy transfer in from P-1 to P-20 is
provided by excitation spectra collected while monitoring emission at 610 nm (Figure 3-
10). The spectrum of the TBT model compound 1b shows that the emission of 610 nm
comes from both 330 nm and 470 nm; however, emission at 610 nm of poly-
chromophores bearing TBT side groups (P-1 to P-20) are mostly originated from OPE
absorption (370 nm). The excitation spectra of P-1 to P-20 match the absorption spectra
of these polymers. These data clearly show that the TBT emission excitation of the OPE
chromophores.
126
Figure 3-10. Excitation spectra of copolymers with both donor and acceptor. P-1 (red
hollow circles), P-5 (green hollow triangles), P-10 (blue hollow inverted triangles), P-20 (brown hollow diamonds) and model compound 1b (red solid circles). The emission wavelength was set at 610 nm.
3.4 Ultrafast Transient Absorption
Ultrafast transient absorption experiments were carried out on the
polychromophores to characterize the dynamics of the intrapolymer energy transfer
process. Figure 3-11 and 3-12 compare the ultrafast transient absorption of the donor-
acceptor co-polymers (P-1 to P-20) with those of the donor only polymer (P-0) and the
TBT acceptor unit as modeled by 1a. The transient spectrum of P-0 (Figure 3-11, red
filled) shows a negative feature (bleach) at 400 nm that results from a combination of
ground state bleach and stimulated OPE emission, as well as an intense excited state
absorption centered near 600 nm. The single wavelength kinetic trace of P-0 (Figure 3-
12, black hollow squares) measured at λ = 665 nm shows biphasic decay. The minor
component (0.20) has a 50 ps lifetime and is attributed to relaxation dynamics within the
singlet excited state, likely due to planarization of the OPE unit. This decay is
127
accompanied by a slight blue shift of the excited state absorption. The major decay
component (0.80) has a 1.2 ns lifetime and is consistent with the excited state lifetime
measured from the fluorescence decay kinetics. As the laser fluence is increased
above 40 J/cm2, the excited state absorption begins to exhibit intensity dependent
kinetics, presumably due to exciton-exciton annihilation events that occur when more
than one excited state is created on each chain. The transient data reported here were
collected at pulse energies below this threshold (25 J/cm2), thus avoiding these
exciton-exciton processes.
Figure 3-11. Transient absorption spectra showing early time (t = 175 fs) comparison
between P-5 (hollow triangles) and the pure donor polymer P-0 (red filled). Also shown are transient spectra at t = 1.15 ns comparing P-5 (hollow squares) with the TBT acceptor moiety (blue filled).
128
Figure 3-12. Transient kinetics from λ = 665 nm for the five polymers. P-0 (black hollow squares), P-1 (red hollow circles), P-5 (green hollow triangles), P-10 (blue hollow inverted triangles), P-20 (brown hollow diamonds).
Examination of the transient absorption of P-5 reveals the importance of energy
transfer to the excited state dynamics when both OPE donor and TBT acceptor moieties
are present on a single chain. Figure 3-11 shows the transient absorption spectrum of
the P-5 donor-acceptor copolymer at 175 fs (hollow triangles). The spectrum is nearly
identical to that of the donor only polymer, P-0 (red filled), indicating that photoexcitation
at 388 nm predominantly creates OPE excited states (OPE*). However, by 1.1 ns
following excitation the transient spectrum of P-5 (hollow squares) has evolved, now
containing absorption and bleach features that are very similar to those of the excited
TBT chromophore (TBT*, compare with transient spectrum of TBT model 1b, blue
filled). These transient absorption results are in accord with the fluorescence spectra,
confirming the occurrence of efficient intrachain OPE to TBT energy transfer.
129
The dynamics of energy transfer can in principle be followed either through the
disappearance of OPE* (i.e. donor decay) or appearance of TBT* (acceptor rise).
However, monitoring the appearance of TBT* via its ground state bleach at = 470 nm
is problematic due to the stimulated emission from OPE* in this spectral region. We
have instead focused our analysis on the decay of OPE* absorption at 665 nm, where
the OPE* stimulated emission and TBT* contributions are minimized.
The single wavelength kinetic trace of P-5 (Figure 3-12, green hollow triangles)
exhibits much faster decay dynamics than donor-only P-0. The accelerated decay is
primarily caused by an additional fast component (τ1 ≈ 3.4 ps), which is assigned to
quenching of OPE* excitons that are formed on a chain in close spatial proximity to a
TBT acceptor unit, and undergo direct energy transfer to the acceptor. In addition to this
fastest component, there is also an intermediate component (τ2 ≈ 26 ps) likely reflecting
two processes: relaxation of the initially formed excited state that is observed in the
donor only polymer, as well as multi-step energy transfer in which the excited state first
has to migrate through a random walk, site-to-site hopping process to a position along
the chain that is in close proximity to an acceptor. The overall timescale for this latter
process will depend on the OPE-OPE hopping rate (khop), the number of hops needed to
reach the trap, as well as the OPE-TBT energy transfer rate (kEnT).
Across the polymer series, a trend of faster OPE* quenching is observed as
loading of the TBT acceptor units increases. The additional fast (τ1) decay component
as well as greater contribution from intermediate rate processes (relative to that seen in
P-0) is found for each of the other co-polymers in the series. Time constants and
130
normalized amplitudes recovered from the triexponential modeling of the 5 polymers (P-
0 – P-20) are summarized in Table 3-1.
Increasing the loading of acceptor chromophores in the copolymer has two
effects. First, the probability of photoexcitation producing excited states within the
quenching radius of an acceptor increases. Second, for those excitons created far from
an acceptor, the number of steps necessary to migrate near enough for quenching to
occur decreases. Both of these effects will result in qualitatively faster quenching of the
excited state, a result which can be seen by comparing the kinetics at 665 nm for the
polymer series in Table 3-1. As the loading increases from 1% to 20%, the amplitude of
the fast component increases from 0.07 to 0.38 with little change in the time constant
itself, consistent with the direct OPE* to TBT energy transfer assignment. The time
constant associated with the intermediate processes (25-50 ps) trends downward as
loading increases, reflecting the shorter amount of time necessary for an excited state
to migrate to the TBT trap.
131
Table 3-1. Photophysical Characteristics of Polymers (P-0 to P-20).
Polymer Mn
/g•mol-1 PDI
TBT
Contenta /%
Emission
Lifetime at
420 nmb /ns
Emission
Lifetime at
600 nmb /ns
Model time constants and amplitudes at
665 nmc
τ1 /ps (A1) τ2 /ps (A2) τ3 /ps (A3)
P-0 29200 1.23 0 2.63 (0.28) 1.00 (0.72)
- (-) -
(-) 44.9 ± 4.7
(0.20 ± 0.01) 1148 ± 35
(0.80 ± 0.01)
P-1 29200 1.17 1 2.02 (0.31) 0.58 (0.69)
10.60 (0.9) 3.04 (0.1)
2.6 ± 0.7 (0.07 ± 0.01)
56.8 ± 4.6 (0.31 ± .01)
960 ± 34 (0.61 ± 0.01)
P-5 28700 1.23 5.5 2.04 (0.24) 0.35 (0.76)
10.10 (1.0) 3.4 ± 0.5
(0.23 ± 0.02) 44.8 ± 3.7
(0.44 ± 0.02) 1240 ± 112
(0.33 ± 0.01)
P-10 28100 1.23 11 1.90 (0.24) 0.36 (0.76)
9.67 (1.0) 2.1 ± 0.2
(0.26 ± 0.01) 27.7 ± 1.5
(0.43 ± 0.01) 1160 ± 66
(0.30 ± 0.01)
P-20 26800 1.23 22 1.78 (0.29) 0.36 (0.71)
8.94 (1.0) 2.0 ± 0.1
(0.38 ± 0.01) 26.1 ± 2.1
(0.33 ± 0.01) 1465 ± 110
(0.29 ± 0.01)
a TBT content determined by UV-Vis absorption. The ratio of absorbance of P-x polymers at 370 nm and 400 nm follows the equation:
(370 nm, P- ) (370 nm, 1a) (370 nm, 1b)
(470 nm, P- ) (470 nm, 1b)
(1 ) =
x
x
I x x
I x
So
(370 nm, 1a)
(370 nm, P- )
(470 nm, 1b) (370 nm, 1a) (370 nm, 1b)
(470 nm, P- )
x
x
xI
I
In which (370 nm, 1a) = 58000 M-1
cm-1
, (370 nm, 1b) = 2640 M-1
cm-1
, and (470 nm, 1b) = 24000 M-1
cm-1
. b Emission lifetime measured by TCSPC. c Measured from single wavelength transient absorption kinetics at = 665 nm.
132
3.5 Molecular Dynamics Simulations
To examine the relationship between energy transfer and polymer structure, we
performed molecular dynamics (MD) simulations of a 30-unit subsection of the P-0, P-5,
P-10, and P-20 polymers with explicit THF solvent (details in Supporting Information).
On average, the nearest neighbor chromophore distance (regardless of whether it was
an OPE-OPE pair or an OPE-TBT pair) was found to be 0.99±0.23 nm. This distance
used in the Förster expression predicts rates that are far greater than those observed
experimentally: a hopping rate of khop = 6.1×1012 s-1 and an energy transfer rate kEnT =
7.5×1013 s-1. However, given that Förster theory tends to overestimate the energy
transfer rates at such short distances (and between chromophores where the point
dipole approximation is no longer valid), quantitative comparison with experimentally
determined rates is invalid unless more elaborate expressions are used for the Coulomb
interaction.168-169 Nevertheless, the average inter-chromophore distance calculated from
MD simulations is well within the Förster radii calculated from spectral overlap and the
OPE quantum yield for both energy transfer (R0 = 6.63 nm) and hopping processes (R0
= 4.37 nm). This suggests that both direct (τ1) and hopping (τ2) routes are efficient
mechanisms of energy migration and transfer. As a result, the polymer motif serves to
efficiently couple donor and acceptor chromophores, creating a network that quickly
shuttles energy and may be a promising model for light harvesting applications.
133
Figure 3-13. Molecular dynamics simulation of the OPE-TBT copolymer. Green: polymer backbone; gray: OPE donor; yellow: TBT acceptor.
3.6 Summary
In summary, energy transport in a light-harvesting polymer consisting of ~ 60
OPE chromophores have been examined. Photoexcitation of OPE sites gives rise to
site-to-site energy transfer and ultimately sensitization of a trap site (TBT) doped into
the polymer at low concentration, on the picosecond time scale and with remarkably
high efficiency. The energy transfer process proceeds via ultrafast neighborhood OPE-
TBT quenching in 2 ~ 4 picoseconds and OPE-OPE energy hopping in 25 ~ 50
picoseconds.
134
3.7 Experimental
3.7.1 Instrumentation and Methods
NMR spectra were measured on a Gemini-300 FT-NMR, a Mercury-300 FT-
NMR, or an Inova-500 FT-NMR. High resolution mass spectrometry was performed on
a Bruker APEX II 4.7 T Fourier Transform Ion Cyclone Resonance mass spectrometer
(Bruker Daltonics, Billerica, MA). Gel permeation chromatography (GPC) analyses were
carried out on a system comprised of a Shimadzu LC-6D pump, Agilent mixed-D
column and a Shimadzu SPD-20A photodioide array (PDA) detector, with THF as eluent
at 1 mL/min flow rate. The system was calibrated against linear polystyrene standards
in THF.
UV-visible measurements were carried out on a Shimadzu UV-1800 dual beam
absorption spectrophotometer using 1 cm quartz cells. Photoluminescence
measurements were obtained on a fluorimeter from Photon Technology International
(PTI) using 1 cm quartz cells. Photoluminescence lifetimes were obtained by using a
single photon counting Fluo Time 100 (Picoquant) Fluorescence Lifetime Spectrometer
and excitation was provided using a PDL 800-B Picosecond Pulsed Diode Laser.
For transient absorption spectroscopy, samples were dissolved in THF to an OD
of between 0.4 and 0.5 in a 2 mm cuvette. Femtosecond pulses were derived from a
Clark-MXR CPA 2210 Ti: Sapphire laser which produces ~ 150 fs pulses centered at
775 nm with a 1kHhz repetition rate. A portion of the output was frequency doubled (to
388 nm) in a BBO and used for photoexcitation of the donor. Low fluences (25 mJ/cm2)
were necessary to achieve linear behavior of the transients. Kinetics was monitored by
a weak continuum probe pulse generated by focusing a small portion of the 775 nm
fundamental into a translating CaF2 window. Spectra were collected at a rate of 1 kHz
135
with pump on and pump off spectra interleaved by mechanical chopping, are chirp
corrected for delay times Δt < 20 ps, and are each the average of 8000 individual pump
on and pump off spectra.
Molecular dynamics (MD) simulations were performed on 30 repeat unit
polymers with 0, 2, 3, and 6 TBT chromophores placed randomly in the structure to
represent the P-0, P-5, P-10, and P-20 copolymers, respectively. Monomer repeat units
with TBT and OPE chromophores were geometrically optimized with the B3LYP DFT
functional and a 6-31G+ basis set as implemented in Gaussian09 version 09a02.170
The structures and DFT determined atomic charges were imported into the Materials
Studio software package for MD. Unit cells consisting of the polymers along with explicit
THF solvent were annealed with 10 temperature cycles ranging from 300 to 500 K using
the Universal Force Field171 as implemented in the Forcite module of Materials Studio.
The final minimum of the annealing cycle was taken as the starting point for subsequent
MD runs. MD on each of the 4 polymers showed the minimum distance between
chromophores was independent of chromophore identity (i.e. TBT-OPE distance or
OPE-OPE distance), so to boost statistical relevance of the dynamics runs, all
neighboring OPE-OPE distances of the P-0 polymer were tracked to find the minimum
distance reported in the manuscript. Molecular dynamics were performed on two
separate P-0 polymers (last and penultimate energy minima of the annealing cycling).
The first structure was run for a total of 20 ns. The second was run for 6 ns.
The “click” reaction can be monitored by GPC and 1H NMR, as presented in
Figure 4-4 and 4-5. After click grafting, the molecular weights increased significantly,
with GPC traces shifting to lower retention time (i.e., higher molecular weight) from
PVBA to P-x polymers. Peaks in the 1H NMR spectrum of P-0 can be divided into
several groups. The first comes from the polystyrene backbone, including two peaks
(6.35 and 6.82 ppm) assigned to phenyl protons in the backbone (a and b in Figure 4-5)
and a peak (5.42 ppm) corresponding to triazolemethylene protons (c in Figure 4-5).
The complete disappearance of 4.27 ppm for the azidomethylene protons in PVBA and
arising of the 5.42 ppm peak for P-x polymers is evidence of complete chromophore
grafting during the azide-alkyne “click” cycloaddition (the 1H NMR spectrum of PVBA is
shown in Figure 3-7). The second group of peaks arises from the n-butylphosphine
protons in the PE2-Pt side groups, at 2.11, 1.60, 1.42, and 0.90 ppm (d, e, f and g in
Figure 4-5). The third group includes resonances corresponding to aromatic protons
from PE2-Pt (6.35 to 7.72 ppm, h - n in Figure 4-5). The first group peaks belonging to
the polymer backbone protons are broad as the grafted side chains limit the free
rotation of backbone, while the second and third group peaks assigned to PE2-Pt side
chain protons are relatively sharp.
From P-0 to P-3, the first and second groups of peaks do not change. However,
new and small peaks appear in the third group, which arise from pyrene protons in Py-
Pt side chains (7.80 - 8.80 ppm, o – w in Figure 4-5). From P-3 to P-100, signals from o
to w become stronger as the Py-Pt content increases, while signals from k to n become
weaker. In P-100, the Py-Pt homopolymer, signals from k to n completely disappeared.
154
Noteworthy, a “ruler” for Py-Pt content is the single peak at 8.71 ppm, which is
assigned to the 10-position proton of pyrene (w in Figure 4-5). This peak arises
significantly along with the Py-Pt content and the peak integration is used to calculate
the actual Py-Pt content in the copolymers. However, the calculation has large error in
P-3, P-5 and P-10, because the single at 8.71 ppm is too small to be integrated
accurately. Accurate calculation relies on UV-visible absorption. For P-20 and P-100,
the Py-Pt contents calculation from NMR agree well with the results from absorption.
155
Figure 4-5. 1H NMR Spectra of Poly-Platinums (P-0 to P-100).
156
4.3 Steady-State Absorption and Emission
Figure 4-6. Absorption of platinum acetylides and Poly-Platinums in THF.
The absorption of the two model platinum acetylides, PE2-Pt model compound
M1 and Py-Pt model compound M2, and six poly-platinums (P-0 to P-100) are
presented in Figure 4-6. The energy donor model M1 has two near-UV peaks at 302 nm
and 350 nm. The acceptor model compound M2 has a near-UV absorption band at 292
nm and three visible absorption bands at 368 nm, 387 nm and 398 nm, and Py-Pt also
has a modulate absorption from 300 nm to 350 nm. P-0 and P-100 is the homopolymer
of PE2-Pt and Py-Pt, respectively and the absorption of the two polymers is nearly
identical to the two model compounds. The extinction coefficients of P-0 and M1 are
similar, which are less than 5% difference (see Table 4-1). It is the same for P-100 and
M2. The extinction coefficients are another evidence of complete grafting during the
azide-alkyne click reaction.
157
The spectra of P-3 to P-20 are dominated by the PE2-Pt based transitions
(especially at 348 nm); however, they show increasing Py-Pt character as the loading of
the lower energy chromophores in the click grafting is increased, which features the
absorption at 398 nm. This feature allows straightforward determination of the Py-Pt
loading in the copolymer from the ratio of the absorbance at 350 and 398 nm.
Considering the absorption of P-3 to P-20 is a linear combination of P-0 and P-100, the
absorption spectra are used to calculate the accurate x value in the polymers. The
absorption at 350 nm arises from both PE2-Pt and Py-Pt side chains; while the
absorption at 398 nm is solely from Py-Pt side chain. So the absorption ratio at 350 nm
and 400 nm will follow Equation 4-2:
(350 nm, P- ) (350 nm, P-0) (350 nm, P-100)
(398 nm, P- ) (398 nm, P-100)
(1 ) =
x
x
I x x
I x
(4-2)
And from Equation 4-2, the x value is calculated as:
(350 nm, P-0)
(350 nm, P- )
(398 nm, P-100) (350 nm, P-0) (350 nm, P-100)
(398 nm, P- )
x
x
xI
I
(4-3)
The calculation results (summarized in Table 4-1) indicate that the fractional
loading of PE2-Pt and Py-Pt corresponds closely to the stoichiometry used in the click
reaction. On the other hand, the simulated absorption spectrum of P-20 (Figure 4-6),
calculated from the absorption spectra and extinction coefficients of P-0 and P-100,
corresponds nicely with the measured P-20 absorption.
Taken together, the absorption data for the model compounds and polymers
allow several conclusions. First, the fractional loading of PE2-Pt and Py-Pt units in the
polymers closely corresponds to the stoichiometry used in the feed for the click
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reactions. Second, the absorption at 350 nm is due almost exclusively to the PE2-Pt
(donor), making it possible to selectively excite this chromophore. Finally, the polymer
spectra are accurately simulated as a linear combination of the spectra of the PE2-Pt
and Py-Pt chromophores, indicating that there is not a strong ground state interaction
among the individual units in the polymers.
Figure 4-7. Comparison of measured (brown) and simulated (violet circle) absorption spectra of P-20. The calculated spectrum was derivate from the absorption spectra and extinction coefficients of P-1 and P-100 based on the equation ε(Calculated P-20) = 0.8 x ε(P-1) + 0.2 x ε(P-100). The results were then normalized to (0, 1) to obtain the spectrum (violet circle) above. The spectra of P-0 and P-100 in this figure are normalized based on their relative molar absorptivities and relative PE2-Pt/Py-Pt content in P-20 copolymer
The phosphorescence spectra of P-0 to P-100 are shown in Figure 4-8. The
phosphorescence measurement was carried out in THF solutions and the solutions
were degassed via bubbling argon for 45 minutes. The phosphorescence of the PE2-Pt
(donor) only polymer P-0 appears at 527 nm, with a shoulder at 562 nm, which is nearly
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identical to that of M1. Similarly, the Py-Pt (acceptor) only polymer P-100 and acceptor
model M2 also have similar phosphorescence spectra, with peaks of 660 and 737 nm.
Figure 4-8. Emission of model compounds and poly-platinums in THF. The solutions
had OD round 0.8 and were deoxygenated by bubbling argon for 45 minutes. The spectra were normalized according to their quantum yields. The excitation wavelength was set at 350 nm for P-0 to P-20 and M1, and 385 nm for P-100 and M2.
As for the copolymers (P-3 to P-20), the excitation wavelength was selected to
be 350 nm, which can selectively excite PE2-Pt side chains. In P-3, which has only 3
percent of Py-Pt loaded onto the copolymer, the phosphorescence from the PE2-Pt
chromophore at 527 nm is quenched ~ 85% relative to the intensity of P-0, and
emission from Py-Pt at 660 nm and 737 nm is evident, indicating the PE2-Pt* to Py-Pt
energy transfer is efficient. The energy transfer efficiency can be calculated with the
quantum yields according to Equation 4-4:58
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PE2-PtEnT
P-0
1
(4-4)
In Equation 4-4, P-0 is the phosphorescence quantum yield of P-0, and PE2-Pt is
quantum yield of phosphorescence emission from PE2-Pt* (480 to 628 nm) in the
copolymers. The quantum yields are listed in Table 4-1. According to Equation 4-4, the
energy transfer efficiency is ~ 86% in P-3. In P-5, the PE2-Pt emission is quenched to a
greater extent, and the energy transfer efficiency from PE2-Pt* to Py-Pt approaches
95%. When Py-Pt content increased to 20% (P-20), the PE2-Pt emission is quenched
almost completely, with EnT ~ 100%.
Figure 4-9. Quantum yields and energy transfer efficiency for poly-platinum copolymers.
Figure 4-9 shows the quantum yields from PE2-Pt and Py-Pt emission,
respectively, and the energy transfer efficiencies in the polymers P-0 to P-20. The PE2-
Pt emission yield decreases sharply from P-0 to P-3, followed by a more gradual decline
from P-5 to P-20. By contrast, the Py-Pt emission yield increases sharply from P-0 to P-
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3; however, it then decreases slightly from P-5 to P-20. The latter trend is likely due to
self-quenching of the Py-Pt chromophore as its concentration in the polymers increases,
possibly due to triplet-triplet annihilation between Py-Pt units. The energy transfer
efficiency increased sharply from P-0 to P-3 (86.3%), and then increased gradually to
approach 100% in P-20. The quantum yield and energy transfer efficiency data are also
listed in Table 4-1.
The phosphorescence emissions clearly demonstrate the triplet-triplet energy
transfer from PE2-Pt (donor) chromophore to Py-Pt (acceptor) chromophore. Additional
evidence for PE2-Pt to Py-Pt energy transfer in from P-3 to P-20 is provided by
excitation spectra, as presented in Figure 4-10, which was collected while monitoring
emission at 660 nm, the Py-Pt emission maximum. The spectrum of the Py-Pt only
polymer P-100 shows that the phosphorecence of 660 nm comes from the three visible
absorption bands of Py-Pt, 368 nm, 387nm and 398 nm. However, emission at 660 nm
of polymers bearing both PE2-Pt and Py-Pt side groups (P-3 to P-20) are mostly
originated from PE2-Pt absorption (~ 347 nm). The excitation spectra of P-3 to P-20
match the absorption spectra of these polymers. These data clearly show that the Py-Pt
emissions are coming from excitation of the PE2-Pt chromophores.
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Figure 4-10. Excitation spectra of poly-platinums. λemission = 660 nm.
163
Table 4-1. Photophysical characteristics of model compounds and poly-platinums.
Compound/
Polymer
Py-Pt
Contenta
/%
Absmax
/nmb
Extinction
Coefficient
/ M-1•cm-1
FLmax
/nmb ϕFL
c Phmax
/nmb
ϕphc Energy
Transfer
Efficiency
/%
ϕPE2-Pt
(480-628
nm)
ϕPy-Pt
(628-850
nm)
ϕtotal
(480-850
nm)
M1 -- 350 84,700 -- -- 527 0.18 -- 0.18 --
M2 --
292 368 387 398
60,900 58,000 66,800 57,100
412 0.002 660 737
-- 0.015 0.015 --
P-0 0 348 89,700 388 <0.0001 527 0.12 -- 0.12 --
P-1 3.2 348 -- 388 <0.0001 527 660 737
0.016 0.023 0.039 86.3
P-5 5.3 348 -- 388 <0.0001 527 660 737
0.0065 0.021 0.028 94.6
P-10 10.3 348 -- 388 <0.0001 527 660 737
0.0025 0.018 0.021 97.9
P-20 21.1 348 -- 388 <0.0001 527 660 737
0.00072 0.015 0.016 99.4
P-100 100
292 368 387
398
66,800 56,100 63,500 54,100
412 0.0013 660 737
-- 0.014 0.014 --
a Determined by UV-visible absorption b Absmax: Ground state absorption maxima; FLmax: Fluorescence emission maxima; Phmax: Phosphorescence emission maxima c With anthracene as quantum yield standard, ϕ = 0.27 in ethanol at room temperature
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4.4 Transient Absorption Characterization
Transient absorption (TA) experiments were carried out on the poly-platinums to
characterize the dynamics of the intrapolymer energy transfer process. Figure 4-11
presents TA spectra for the two model compounds M1 and M2. The spectrum of M1
(Figure 4-11A) shows a negative feature (bleach) at 365 nm that results from ground
state bleach of M1, as well as an intense excited state absorption centered near 580
nm. Meanwhile, the TA spectrum of M2 (Figure 4-11B) shows strong negative bleach at
~ 387 nm, a modulate intense excited state absorption at ~ 443 nm due to T1-Tn
absorption, and a broad absorption centered at 548 nm, which may come from the
excimer as pyrene moiety exists in the compound. The TA spectra of M1 and M2 are
similar to previously reported spectra of platinum acetylides with similar structures.149
Figure 4-11. Transient absorption spectra of model compounds. A) M1 and B) M2.
Figure 4-13 compares the transient absorption of the donor-acceptor co-
polymers (P-3 to P-20) with those of the donor only polymer (P-0) and the acceptor only
polymer (P-100). The TA spectra of P-0 (Figure 4-12, black) and P-100 (Figure 4-12,
pink) show similar features as those of M1 and M2.
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Figure 4-12. Transient absorption spectra of copolymers (P-3 to P-20) at different time
and comparison with homopolymers (P-0 and P-100)
Examination of the transient absorption of copolymers P-3 to P-20 clearly reveals
the presence of energy transfer when both PE2-Pt donor and Py-Pt acceptor moieties
are present on a single chain. Figure 4-12a shows the transient absorption spectrum of
the P-3 donor-acceptor copolymer at 60 ns (red) and 2000 ns (blue). The spectrum at
60 ns is nearly identical to that of the donor-only polymer, P-0 (black), indicating that
photoexcitation at 355 nm predominantly creates PE2-Pt excited states (PE2-Pt*);
however, a small peak around 460 nm arises, which is coming from Py-Pt excited states
(Py-Pt*), suggesting certain amount of triplet energy have transferred from PE2-Pt* to
Py-Pt within 60 ns. In addition, by 2000 ns following excitation the transient spectrum of
P-3 (blue) has evolved, the spectrum is dominated by the positive absorption around
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450 nm that are coming from the T1-Tn absorption of the excited Py-Pt*. The 543 nm
absorption peak does not show as in the copolymer with low Py-Pt content, excimer is
difficult to form.
Investigations of other copolymers, P-5 to P-20 reveal similar phenomenon as P-
3. Note the delay time needed for the 460 nm small shoulder to appear is getting shorter
from P-3 to P-20, which means the energy transfer from donor to acceptor becomes
faster as Py-Pt content increases.
4.5 Time-Resolved Emission
Time-resolved emission can be characterized by both time-correlated single
photon counting (TCSPC) and transient absorption (TA). The working mechanisms for
these two instrumentations are different. TCSPC counts photons emitted from T1 to S0
transition directly. On the other hand, TA deals with excitons staying in T1 state at
different times, thus studying T1 to S0 transition indirectly.184 Both methodologies were
applied to study the kinetics of energy transfer from PE2-Pt* to Py-Pt.
Figure 4-13A shows emission decays at 520 nm (peak value of PE2-Pt
phosphorescence emission) of PE2-Pt containing polymer P-0 to P-20. The decay of
PE2-Pt only polymer P-0 is biexponential with a fast decay (τ1 = 1.89 µs) and a slow
decay (τ2 = 62 µs); the two components have nearly equal amplitude. As 3% of Py-Pt is
loaded, the lifetime of fast decay decreases by 30% (τ1 = 1.22 µs) and the amplitude
increases up to 0.90. As the acceptor content increases, the time constants for both
decay components are shortened; and the time constant of the fast decay (τ1) deceases
to 360 ns for P-20, with a amplitude of 0.96. The trend is more clear in the expanded
view from 0 to 100 µs, as displaced in Figure 4-11B. The lifetimes and amplitudes
obtained from the time-resolved emission are listed in Table 4-2.
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Figure 4-13. Phosphorescence decay of poly-platinums at 520 nm. A) Complete decay
from 0 to 500 µs. B) Decay from 0 to 100 µs.
The decrease of the fast decay time constant and increasing of its amplitude
clearly indicate the energy transfer from PE2-Pt* to Py-Pt, and this quenching process is
a dynamic process. The increasing of Py-Pt (acceptor) in the copolymer speeds up the
quenching process. From the lifetime values, we could conclude the energy transfer
happens with in 1 µs.
More detailed study about the energy transfer within the 1 µs timescale was
further studied by transient absorption, by following the single wavelength at λ = 600 nm.
This wavelength is close to the peak value of T1 to Tn absorption of PE2-Pt. The decays
of polymers are presented in Figure 4-14.
Interestingly, comparing to the long-time scale decay, biphasic decays are also
observed in the polymers, with much shorter lifetimes when examining the decay
kinetics within 1 µs. The decay of P-0 exhibits a fast component (τ1 = 92.7 ns and A1 =
0.43) and an intermediate component (τ2 = 817 ns and A2 = 0.57). As the loading of Py-
Pt increases from 3% to 20%, the time constants of fast components (τ1) decreases
from 53.0 ns to 15.9 ns, with the amplitude increased from 0.56 to 0.75. Meanwhile,
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time constants of the second component (τ1) decrease from 395 ns to 222 ns, with a
decreased amplitude from 0.44 to 0.25. The first component (τ1) can be assigned to the
direct energy transfer from nearest neighbor PE2-Pt* to Py-Pt, with a timescale from 10
to 50 ns. The second component (τ2) is attributed to energy migration through a random
walk, site-to-site hopping process of PE2-Pt exited energy to a position along the chain
that is in close proximity to an acceptor.
Figure 4-14. Transient kinetics from λ = 600 nm for five polymers P-0 to P-20 on the
timescale of 0 to 0.92µs.
Increasing the loading of acceptor chromophores in the copolymer has two
effects. First, the probability of photoexcitation producing excited states next to an
acceptor increases. Second, for those excitons created far from an acceptor, the
number of steps necessary to migrate near enough for quenching to occur decreases.
Both of these effects will result in qualitatively faster quenching of the excited state,
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which agrees with the amplitude change for the direct energy transfer and energy
migration processes.
Table 4-2. Liftimes of poly-platinums.
Polymer
Long-time Scale (measured by TA)
Short-time Scale (measured by TA)
τ1 /μs (A1) τ2 /μs (A2) τ1 /ns (A1) τ2 /ns (A2)
P-0 1.89 (0.49)
61.6 (0.51)
92.7 (0.43)
817 (0.57)
P-3 1.29 (0.90)
43.6 (0.10)
53.0 (0.56)
395 (0.44)
P-5 1.11 (0.92)
36.5 (0.77)
31.5
(0.54) 295
(0.46)
P-10 0.48 (0.95)
31.1 (0.05)
22.6
(0.64) 252
(0.36)
P-20 0.36 (0.96)
24.0 (0.04)
15.9 (0.75)
222 (0.25)
4.6 Energy Transfer Pathway
In the system with triplet state, the energy transfer may have two distinct
mechanisms. After the donor in the copolymer is excited selectively to produce 1(PE2-
Pt), as show in Figure 4-14 path 1, the first mechanism involves rapid singlet energy
transfer from 1(PE2-Pt) to 1(Py-Pt) (Figure 4-15 path 2), as described in Chapter 3.
Following singlet transfer, 1(Py-Pt) can relax either radiatively (fluorescence) or via
intersystem crossing to produce 3(Py-Pt) (paths 3 and 4 in Figure 4-15).
The second mechanism, which is triplet energy transfer, involves intersystem
crossing on PE2-Pt unit to produce 3(PE2-Pt) (Figure 4-14 path 5), followed by
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intrachain triplet energy transfer from 3(PE2-Pt) to 3(Py-Pt). Radiative decay of 3(Py-Pt)
emits phosphorescence photons.
To determine which mechanism the energy transfer in the copolymer follows, the
relative rates of intersystem crossing, singlet energy transfer and triplet energy transfer
should be considered. Previously studies reveal that the intersystem crossing of
platinum acetylides is in the timescale less than 10 ps.149, 180 Studies in Chapter 3
indicate the singlet energy transfer lies in 20-50 ps. Therefore, energy transfer following
the first mechanism, involving singlet energy transfer, should occur in picoseconds
range. Considering the energy transfer in P-3 to P-20 has time constants in
nanoseconds range, the first mechanism is ruled out as the energy transfer in the
copolymers is much slower than intersystem crossing and singlet transfer. Thus we can
conclude the energy transfer in the copolymers follows triplet-triplet energy transfer
mechanism.
Figure 4-15. Jablonski Diagram for energy transfer in copolymers. The energy level is
calculated from emission peak values.
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4.7 Summary
In summary, a series of polymeric arrays consisting of ~ 60 platinum acetylide
chromophores have been synthesized with RAFT-SN2-“click” strategy, and energy
transport in the polymers has been examined. Photoexcitation of PE2-Pt sites gives rise
to site-to-site triplet energy transfer and ultimately sensitization of a trap site (Py-Pt)
doped into the polymer at low concentration, on the nanosecond time scale and with
remarkably high efficiency. The energy transfer process proceeds following triplet-triplet
transfer pathway, via a fast neighborhood (PE2-Pt) – (Py-Pt) quenching in 10 ~ 50
nanoseconds and (PE2-Pt) – (PE2-Pt) energy hopping in 200 ~ 400 nanoseconds.
4.8 Experimental
4.8.1 Instrumentation and Methods
NMR spectra were measured on a Gemini-300 FT-NMR, a Mercury-300 FT-
NMR, or an Inova-500 FT-NMR. High resolution mass spectrometry was performed on
a Bruker APEX II 4.7 T Fourier Transform Ion Cyclone Resonance mass spectrometer
(Bruker Daltonics, Billerica, MA). Gel permeation chromatography (GPC) analyses were
carried out on a system comprised of a Shimadzu LC-6D pump, Agilent mixed-D
column and a Shimadzu SPD-20A photodioide array (PDA) detector, with THF as eluent
at 1 mL/min flow rate. The system was calibrated against linear polystyrene standards
in THF.
For UV-visible absorption measurements, samples were dissolved in THF and
were carried out on a Shimadzu UV-1800 dual beam absorption spectrophotometer
using 1 cm quartz cells. Photoluminescence measurements were obtained on a
fluorimeter from Photon Technology International (PTI) using 1 cm quartz cells. For
172
phosphorescence measurement the sample solutions were degassed via bubbling
argon for 45 minutes.
Luminescence lifetimes were obtained with a multichannel scaler/photon counter
system with a PicoQuant FluoTime 100 Compact Luminescence Lifetime
Spectrophotometer. A high-performance Coherent CUBE diode laser provided the
excitation at 375 nm (P < 10 mW). The laser was pulsed using an external Stanford
Research Systems DG535 digital decay and pulse generator with four independent
delay channels. At least four narrow band pass filters were used for measurements
followed by global fit processing (FluoroFit software). Decays were obtained using the
a Ground state absorption maxima. b Emission spectra, obtained by excitation at 455 nm. c With Ru(bpy)3Cl2 as quantum yield standard, ϕ = 0.0379 in air-saturated water. d Emission lifetimes, from time-correlated single photon counting (TCSPC), and ⟨τ⟩ is median lifetime calculated as ⟨τ⟩ =
ΣAiτi.
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5.4 Amplified Quenching
The concept “amplified quenching” was first studied by Swager and coworkers in
main-chain conjugated poly (phenylacetylene)s (PPEs).48-49 When a quencher is binding
to PPEs with host-guest interaction, the Stern-Volmer constant is much larger than that
in the interaction between quencher and small molecular monomer. Whitten, Schanze
and coworkers expanded this concept to conjugated polyelectrolytes (CPEs). CPEs can
be quenched by small amount of oppositely charged quencher ions. This process has
been attributed to two main factors: (1) the ion-pairing between the charged quencher
ion and the polyelectrolyte chain effectively to increase the local concentration of the
quencher; and more important (2) the exciton in the polyelectrolyte are able to undergo
rapid diffusive transport along the polymer chain, in effect increasing the effective
sphere of action of the quencher ion.50
Amplified quenching can also be expanded to polyelectrolytes with non-
conjugated backbone and pendant chromophores, and it is an effective method to
examine the energy migration (by hopping) along the non-conjugated chain. The
amplified quenching experiments of the two polymers with the Cl- counter ion (P1-Cl
and P2-Cl) were taken in water with sodium 9,10-anthraquinone-2,6-disulfonate (AQS)
as the quencher.
Figure 5-11 demonstrates the quenching studies of P1-Cl and P2-Cl. The
polymer concentrations were controlled at 10 µM in water based on the repeat units and
quenched by addition of AQS. The quenching of both polymer emissions was similar.
Both quenching were very efficient, with 50% quenching (I0/I = 2) were observed around
0.4 µM of AQS, meaning one AQS molecule could quench around 12 Ru chromophores.
The Stern-Volmer plots were also obtained, which were linear until around 75% of the
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initial emission intensities were quenched (I0/I = 3.8), and Stern-Volmer constants
around 2.8 x 106 M-1 were obtained (Table 5-1). Above this value, the plots curved
upwards, indicating higher concentration of opposite charged AQS may induce
aggregation of Ru arrays. In comparison, the quenching of model complex 2-Cl was
also studied (Figure 5-11, right) and a small Stern-Volmer constants of 2.67 x 104 M-1
was obtained. This comparison reveals that there is significant amplification of
quenching in both polymers, with a ~100 fold magnification. This result clearly indicates
the MLCT exciton diffusions along the non-conjugated polymer backbone. In addition,
end-groups do not have significant effect on the amplified quenching behavior.
Figure 5-11. Emission quenching of polymers (P1-Cl, left, and P2-Cl, middle) and model
complex (2-Cl, right).
198
Figure 5-12. Stern-Volmer plots for emission quenching of P1-Cl (black squares), P2-Cl
(red circles) and 2-Cl (blue triangles).
5.5 Surface Absorption on Titanium Dioxide Surface
The main objective of installing carboxylic acids on Ru-functional polymer end-
group is to utlize them to anchor to surfaces. Therfore, their ability of P2 to absorb onto
TiO2 surface and inject electrons into TiO2 was investigated with n-type rutile TiO2 (110)
single crystals.
Single crystals of rutile TiO2 are good candidates for the study of the morphology
-structure interactions as dyes, conjugated polyelectrolytes and quantum dots show
efficient electron injection into the single crystals.209-214 Morphology studies were carried
out to correspond to the photocurrent results with the surface coverage. The unmodified
single crystals of TiO2 (110) are flat with terraces of different size present. Morphology-
photocurrent interactions were also successfully investigated by Ginger et al. for the
bulk heterojunction materials.215-216 Nanocrystalline TiO2 will also be used to correlate
the efficiency of the electron injection with the data obtained for single crystals. Atomic
199
force microscopy (AFM) results for the flat rutile single crystals show the presence of
clean terraced surfaces without impurities present. Different magnification images are
shown to clearly demonstrate the homogeneous nature of the surface. The height of the
terraces is around 100 - 200 pm with the width of around 60 nm, as illustrated in Figure
5-16.
Figure 5-13. Non-contact tapping mode AFM images of TiO2 (110) with different resolutions (A-C) and cross-section analysis of C (D).
Addition of the dyes to the surface causes appearance of the certain features
with range of sizes at the TiO2 (110) surface. Hereby two series of studies have been
carried out. The first is to soap single crystal TiO2 into P2-Cl in methanol solutions with
200
different concentrations with a constant time (12 hours). The second study is to
immerse single crystal TiO2 into P2-Cl in methanol solution with a constant
concentration (1 μg/mL) for different soaking times. After removing single crystal TiO2
out of solution, the substrates were rinsed with methanol, dried and characterized with
AFM and photo-current measurement (characterized with internal photon to current
efficiency, IPCE).
Morphology change of P2-Cl on TiO2 (110) surface along with polymer
concentration is demonstrated in Figure 5-14. At lower concentration (10 μg/mL), single
particles are formed along the TiO2 (110) terraces (Figure 5-14A and B). Most particles
have sized from 1 to 2 nm, and small amount of particles have larger sizes around 8 nm,
as demonstrated in cross-section analysis (Figure 5-15A and B). The small particle
sizes are consistent with the radius for the monomer units, suggesting there may be
single polymer chains lying down on TiO2 (110) surface. As polymer concentration
increases (30 – 80 μg/mL), polymer layer begins to form and covers TiO2 surface
(Figure 5-14 C - E). When polymer concentration is larger than 150 μg/mL, larger
aggregates of P2-Cl start to form (Figure 5-14 F – H), and the sizes of 5 – 10 nm are
observed (Figure 5-15H).
These polymer “films” on TiO2 were also characterized via photoelectrochemical
study. The IPCE results show the presence of a substantial signal which is close to the
ground-state absorption (Figure 5-16A). Increase of the concentration of the solution
causing increase of the photocurrent response (internal photon to current efficiency,
IPCE) as a consequence of higher surface coverage; however, there is saturation
201
noticed at certain concentration (Figure 5-16B), which may be caused by the formation
of the monolayer.
202
Figure 5-14. Non-contact tapping mode AFM images of P2-Cl deposited on TiO2 (110) surface from solutions of different concentrations. (A) and (B) 10, (C) 30, (D) 50, (E) 80, (F) an (G) 150 , (H) 600 μg/mL. MeOH was the solvent used for deposition with a dipping time of 12 hours; (B) and (G) are zoom in version of (A) and (F).
Figure 5-15. Cross-section analysis for AFM images for Figure 5-17A, B and H.
Figure 5-16. (a) IPCE spectra for a TiO2 (110) electrode dipped into MeOH with various
concentrations of P2; numbers by the arrow indicates the concentrations used for deposition in μg/mL; (b) IPCE values as a function of the dipping solution concentration for curves shown in A.
When immersing single crystal TiO2 in dilute P2-Cl solution (1 μg/mL), the
amount of polymer seems to be insufficient to cover the whole TiO2 surface to form the
monolayer, so small particles are formed. Cross-section analysis of the AFM image in
initial stage (Figure 5-17A) reveals that the height polymer particle is around 1 nm,
suggesting single polymer chains lying down on TiO2 (110) surface. As the dipping time
increases, the polymer aggregates grow around the small particle “nucleus” to form
203
larger particles. The maximum IPCE value (at 456 nm) also increases with dipping time
as more polymers are absorbed onto TiO2 surface (Figure 5-17F).
Figure 5-17. AFM image of the P2 polymer molecules at the surface of TiO2 (110) from
solutions of 1 μg/ml for different dipping times: (a) 12 hours, (b) 24 hours, (c) 36 hours, (d) 48 hours; (e) cross-section of (a); (f) IPCE values as a function of the dipping time.
The AFM images of P2-Cl onto single crystal TiO2 along with the photocurrent
action characterizations clearly demonstrated that the Ru-functional polymer is able to
204
anchor to TiO2 surface and inject electrons to TiO2 to produce current. The adsorption of
the polymer onto TiO2 initials with single particles, and then the polymer will cover the
whole surface to form a monolayer. After that, more polymer may aggregate onto the
monolayer; however, the photocurrent reaches a limit after the monolayer is formed,
because the electrons produced from aggregates above the monolayer may not be able
to inject into TiO2.
5.6 Solar Cell Performance Characterization
As there is a saturation effect for single crystal TiO2 after polymer monolayer is
formed (Figure 5-16), nanocrystalline TiO2 film (with ~20 nm particle size) on fluorine-
doped tin(IV) oxide (FTO) sustrate was employed to fabricate the dye-sensitized solar
cells (DSSCs).
Figure 5-18a illustrates the photocurrent action spectrum (IPCE) of the polymer-
sensitized solar cells obtained using monochromatic illumination under short circuit
conditions. The trend of peak efficiency is largely consistent with IPCE spectra of a TiO2
(110) electrode. The solar cell has a peak IPCE value of 1.76% at 460 nm. This value is
25 times larger than that from single crystal TiO2, indicating much better polymer
absorption on nanocrystalline TiO2 film.
The J-V characteristics under AM1.5 illumination (100 mW/cm2) of the polymer
sensitized DSSCs are shown in Figure 5-18b. The performance of DSSCs are
characterized with of short-circuit current density (Jsc), open-circuit voltage (Voc), fill
factor (FF), and overall power conversion efficiency (ηcell). For the cell made from P2-Cl,
it has relative high open-circuit voltage (Voc = 464 mV) and fill factor (FF = 0.62);
however, the short-circuit current density is low (Jsc = 0.34 mA/cm-2), resulting a low
overall power conversion efficiency (ηcell =0.10%).
205
Figure 5-18. Photocurrent action spectrum (IPCE, A) and J-V curve (B) of DSSC made
from P2-Cl and nanocrystalline TiO2.
Although the IPCE and overall power conversion efficiency of the cell from the
polymer P2-Cl is relatively low, the characterizations have clearly shown the Ru-
functional polymer can act as a polymeric dye in DSSCs. Further efforts, including
tuning polymer chain length, adjusting structures of pendant ruthenium complex and
optimizing DCCS fabrication technique, etc, should be made to improve the solar cell
performances.
5.7 Summary
In this chapter, we have reported the preparation and characterization of
ruthenium(II)-functional polymer with acid end-group. The polymer was prepared via
NMP-SN2-click strategy, and featured a well-defined polystyrene backbone, pedant
ruthenium(II) chromophores and a triacid end-group. The polymer shows typical
absorption and emission as the Ru(bpy)32+ complex, and it also exhibits the amplifed
quenching effect. Absorption of the acid-end-group polymer, P2-Cl, onto single crystal
TiO2 has been characterized by AFM and photocurrent action spectra. The results
indicate Ru-functional polymer is able to anchor to TiO2 surface with the acid end-
206
groups and inject electrons to TiO2 to produce current. DSSCs made from P2-Cl and
nanocrystalline TiO2 shows photon-to-current conversion with a low overall efficiency.
Further efforts, including tuning polymer chain length, adjusting structures of pendant
ruthenium complex and optimizing DCCS fabrication technique, etc, should be made to
improve the solar cell performances.
5.8 Experimental
5.8.1 Instrumentation and Methods
NMR spectra were measured on an Inova-500 FT-NMR. High resolution mass
spectrometry was performed on a Bruker APEX II 4.7 T Fourier Transform Ion Cyclone
Resonance mass spectrometer (Bruker Daltonics, Billerica, MA).
For UV-visible absorption measurements, samples were dissolved in acetonitrile
or methanol and were carried out on a Shimadzu UV-1800 dual beam absorption
spectrophotometer using 1 cm quartz cells. Photoluminescence measurements were
obtained on a fluorimeter from Photon Technology International (PTI) using 1 cm quartz
cells. For phosphorescence measurement the sample solutions were degassed via
bubbling argon for 45 minutes.
Luminescence lifetimes were obtained with a multichannel scaler/photon counter
system with a PicoQuant FluoTime 100 Compact Luminescence Lifetime
Spectrophotometer. A high-performance Coherent CUBE diode laser provided the
excitation at 375 nm (P < 10 mW). The laser was pulsed using an external Stanford
Research Systems DG535 digital decay and pulse generator with four independent
delay channels. At least four narrow band pass filters were used for measurements
followed by global fit processing (FluoroFit software). Decays were obtained using the
15 mg of P1-PF6 was dissolved in acetone, and a saturated tertraammonium
chloride (30 mg) in acetone solution was added dropwise. The red precipitate was
collected, washed with acetone for several times and dry in vacuo to yield red-brown
polymer (11 mg).
218
CHAPTER 6 CONCLUSION
π-Conjugated polymers and oligomers have attracted a lot of interest; it is not
only due to their optical and electronic properties, but also because π-conjugated
organic materials provide the possibility for device performance with freedom to control
over materials, and thereby, device properties. These control are from synthesis of
novel designed molecules, to appropriate morphologies and interfacial interaction in
device architectures. Based on the structures, the π-conjugated organic materials can
be divided into two categories, π-conjugated polymers and small π-conjugated
molecules (or oligomers).
One of the most attractive aspects of conjugated polymers is their solution-
processbility. However, the reproducibility from batch to batch is difficult to control, due
to the broad molecular weight distribution nature of condensation polymerization.
Conjugated oligomers, on the other hand, offer more precisely defined structures,
simpler purification methods, easier modification and functionalization, no end-group
contamination, and more reproducible synthesis. However, the poor film-forming ability
is the disadvantage for the small molecular conjugated oligomers.
The side-chain conjugated polymers, which structurally feature a non-conjugated
and flexible polymer backbone with pendant conjugated chromophores, combine the
intrinsic film-forming and mechanical properties of polymers and well-defined electronic,
photonic, and morphological properties of the monodisperse oligomer moieties. In
addition, the generally low solubility of the oligomers can be improved significantly.
With the development of controlled radical polymerization methodologies, it is
possible to precisely control the length and molecular weight distribution of the polymer
219
backbone and thereby, architectures of side-chain conjugated polymers. In this
dissertation, a post-polymerization modification synthetic strategy was focused and
employed to prepare side-chain conjugated polymers with well-defined structures. The
strategy involves controlled radical polymerization, SN2 substitution and copper(I)
catalyzed azide-alkyne cycloaddition, which abbreviates as CRP- SN2- “click”.
In this dissertation it is demonstrated that the CRP- SN2- “click” strategy is a
versatile route to prepare side-chain conjugated light-harvesting polymers. The
monomers utilized to construct the “clickable” precursor used in this dissertation (GMA
and VBC) were commercially available, inexpensive and easy to purify. When a single
reactive polymer precursor (such as PVBA in Chapter 3 and Chapter 4) is prepared, a
diverse library of functional polymers with identical chain lengths and chain-length
distributions can be generated based upon the parent precursor. The functional
chromophores can be organic (Chapter 3 and 4) and organometallic compounds (such
as platinum acetylides and ruthenium complexes, Chapter 2 and Chapter 5). The
resulting side-chain conjugated light-harvesting polymers may have one or several
different chromophores. In polymers with different chromophores, the chromophore
ration in the polymers closely corresponds to the stoichiometry used in the feed for the
click reactions (Chapter 3 and 4). Different CRP techniques (such as RAFT and NMP)
can be employed to synthesize the well-defined polymer backbone, and end-group
modification can be achieved through functional monomers (Chapter 5).
Singlet and triplet energy transfer has been studied in copolymers with both
donor and accepter in the same polystyrene backbone. The energy transfer from donor
to accept was characterized employing both time-resolved and steady-state
220
fluorescence spectroscopy, as well as time-resolved transient absorption spectroscopy.
The dynamics of both singlet and triplet energy transfer were explored. The ultrafast
energy transfer from donor (OPE) to acceptor (TBT) occurs within 50 picoseconds with
remarkably high efficiency. There were two energy migration processes existing:
ultrafast neighboring OPE-TBT quenching within 2 - 4 ps and OPE-OPE hopping within
25 - 50 ps. In a similar approach, the triplet-triplet energy transfer from donor (PE2-Pt)
to (Py-Pt) was found to be also very efficient, occurring within 50 ns. The singlet energy
transfer follows both Föster and Dexter mechanisms, while the triplet energy transfer
only follows Dexter mechanism.
The applications of light-harvesting polymer made from the CRP- SN2- “click”
strategy were also investigated. A polyacrylate with pendant nonlinear absorption (NLA)
chromophores was prepared via the RAFT-SN2-“click” synthetic strategy. Platinum
acetylides that undergo NLA via both two-photon absorption (TPA) and exited-state
absorption (ESA) mechanisms were utilized as chromophores attached to clickable
polyacrylate backbone. The resulting polymer exhibits similar photophysical properties
as platinum acetylide precursor, including steady-state absorption and emission, triplet-
triplet transient absorption, and nonlinear absorption properties. In addition, the resulting
polymers can be easily drop- or spin- coated to afford optically transparent film.
A ruthenium(II)-functional polymer with carboxylic acid bearing end-group was
prepard via NMP-SN2-click strategy. The polymer shows typical absorption and
emissiton as Ru(bpy)32+ complex and also exhibits amplifed quenching effect.
Absorption of the acid-end-group polymer onto single crystal TiO2 has been
characterized by AFM and photocurrent action spectra. The results indicate Ru-
221
functional polymer is able to anchor to TiO2 surface with the acid end-groups and inject
electrons to TiO2 to produce current. DSSCs made from the acid-end-group Ru-
functional polymer and nanocrystalline TiO2 shows photon-to-current conversion with a
low overall efficiency. Further efforts, including tuning polymer chain length, adjusting
structures of pendant ruthenium complex and optimizing DCCS fabrication technique,
etc, should be made to improve the solar cell performances.
In summary, a versatile methodology to prepare side-conjugated light-harvesting
polymer has been developed and employed in the preparation of several families of
light-harvesting polymers. These polymers can be utilized in both photophysical studies
and materials applications.
222
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BIOGRAPHICAL SKETCH
Zhuo Chen was born in Gaoan, Jiangxi, a small town in east China. At the
age of 16, he went to Fudan University in Shanghai, where he earned his
Bachelor of Science degree in Macromolecular Materials and Engineering in the
summer of 2006. He also studied in the University of Hong Kong as exchange
student for half a year in 2005.
Zhuo then attended the University of Florida for graduate school in 2006.
He first worked with Prof. Charles Beatty in Department of Materials Science and
Engineering, where he got his Master of Science degree in December 2007.
During the 16 months in MSE department, Zhuo studied modification and
strengthen of thermoplastic elastomers with the aid of super-critical carbon
dioxide.
Zhuo transferred to Department of Chemistry in 2008 in the area of
organic chemistry under the supervision of Prof. Kirk Schanze. His research
focused on preparation of well-defined polymeric arrays with pendant organic
and organometallic chromophores for fundamental energy transfer research and
functional materials application. Zhuo was awarded Butler Polymer Research
Award in 2012. He received his Ph.D. degree from the University of Florida in the